Merge branch 'feature-gradle' into develop

34 files changed, 10719 insertions, 0 deletions

diff --git a/src/main/java/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java b/src/main/java/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java
new file mode 100644
index 0000000..bdc3a2e
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/DefaultPrefixRepetitionRule.java
@@ -0,0 +1,95 @@
+/**
+ * @since 2020-08-26
+ */
+package org.unitConverter.converterGUI;
+
+import java.util.List;
+import java.util.function.Predicate;
+
+import org.unitConverter.unit.SI;
+import org.unitConverter.unit.UnitPrefix;
+
+/**
+ * A rule that specifies whether prefix repetition is allowed
+ *
+ * @since 2020-08-26
+ */
+enum DefaultPrefixRepetitionRule implements Predicate<List<UnitPrefix>> {
+	NO_REPETITION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			return prefixes.size() <= 1;
+		}
+	},
+	NO_RESTRICTION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			return true;
+		}
+	},
+	/**
+	 * You are allowed to have any number of Yotta/Yocto followed by possibly one
+	 * Kilo-Zetta/Milli-Zepto followed by possibly one Deca/Hecto. Same for
+	 * reducing prefixes, don't mix magnifying and reducing. Non-metric
+	 * (including binary) prefixes can't be repeated.
+	 */
+	COMPLEX_REPETITION {
+		@Override
+		public boolean test(List<UnitPrefix> prefixes) {
+			// determine whether we are magnifying or reducing
+			final boolean magnifying;
+			if (prefixes.isEmpty())
+				return true;
+			else if (prefixes.get(0).getMultiplier() > 1) {
+				magnifying = true;
+			} else {
+				magnifying = false;
+			}
+			
+			// if the first prefix is non-metric (including binary prefixes),
+			// assume we are using non-metric prefixes
+			// non-metric prefixes are allowed, but can't be repeated.
+			if (!SI.DECIMAL_PREFIXES.contains(prefixes.get(0)))
+				return NO_REPETITION.test(prefixes);
+			
+			int part = 0; // 0=yotta/yoctos, 1=kilo-zetta/milli-zepto,
+								// 2=deka,hecto,deci,centi
+			
+			for (final UnitPrefix prefix : prefixes) {
+				// check that the current prefix is metric and appropriately
+				// magnifying/reducing
+				if (!SI.DECIMAL_PREFIXES.contains(prefix))
+					return false;
+				if (magnifying != prefix.getMultiplier() > 1)
+					return false;
+					
+				// check if the current prefix is correct
+				// since part is set *after* this check, part designates the state
+				// of the *previous* prefix
+				switch (part) {
+				case 0:
+					// do nothing, any prefix is valid after a yotta
+					break;
+				case 1:
+					// after a kilo-zetta, only deka/hecto are valid
+					if (SI.THOUSAND_PREFIXES.contains(prefix))
+						return false;
+					break;
+				case 2:
+					// deka/hecto must be the last prefix, so this is always invalid
+					return false;
+				}
+				
+				// set part
+				if (SI.YOTTA.equals(prefix) || SI.YOCTO.equals(prefix)) {
+					part = 0;
+				} else if (SI.THOUSAND_PREFIXES.contains(prefix)) {
+					part = 1;
+				} else {
+					part = 2;
+				}
+			}
+			return true;
+		}
+	};
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/DelegateListModel.java b/src/main/java/org/unitConverter/converterGUI/DelegateListModel.java
new file mode 100644
index 0000000..b80f63d
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/DelegateListModel.java
@@ -0,0 +1,242 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.List;
+import java.util.ListIterator;
+
+import javax.swing.AbstractListModel;
+
+/**
+ * A list model that delegates to a list.
+ * <p>
+ * It is recommended to use the delegate methods in DelegateListModel instead of the delegated list's methods because
+ * the delegate methods handle updating the list.
+ * </p>
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-14
+ * @since v0.1.0
+ */
+final class DelegateListModel<E> extends AbstractListModel<E> implements List<E> {
+	/**
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	private static final long serialVersionUID = 8985494428224810045L;
+
+	/**
+	 * The list that this model is a delegate to.
+	 * 
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	private final List<E> delegate;
+
+	/**
+	 * Creates an empty {@code DelegateListModel}.
+	 * 
+	 * @since 2019-04-13
+	 */
+	public DelegateListModel() {
+		this(new ArrayList<>());
+	}
+
+	/**
+	 * Creates the {@code DelegateListModel}.
+	 * 
+	 * @param delegate
+	 *            list to delegate
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	public DelegateListModel(final List<E> delegate) {
+		this.delegate = delegate;
+	}
+
+	@Override
+	public boolean add(final E element) {
+		final int index = this.delegate.size();
+		final boolean success = this.delegate.add(element);
+		this.fireIntervalAdded(this, index, index);
+		return success;
+	}
+
+	@Override
+	public void add(final int index, final E element) {
+		this.delegate.add(index, element);
+		this.fireIntervalAdded(this, index, index);
+	}
+
+	@Override
+	public boolean addAll(final Collection<? extends E> c) {
+		boolean changed = false;
+		for (final E e : c) {
+			if (this.add(e)) {
+				changed = true;
+			}
+		}
+		return changed;
+	}
+
+	@Override
+	public boolean addAll(final int index, final Collection<? extends E> c) {
+		for (final E e : c) {
+			this.add(index, e);
+		}
+		return !c.isEmpty(); // Since this is a list, it will always change if c has elements.
+	}
+
+	@Override
+	public void clear() {
+		final int oldSize = this.delegate.size();
+		this.delegate.clear();
+		if (oldSize >= 1) {
+			this.fireIntervalRemoved(this, 0, oldSize - 1);
+		}
+	}
+
+	@Override
+	public boolean contains(final Object elem) {
+		return this.delegate.contains(elem);
+	}
+
+	@Override
+	public boolean containsAll(final Collection<?> c) {
+		for (final Object e : c) {
+			if (!c.contains(e))
+				return false;
+		}
+		return true;
+	}
+
+	@Override
+	public E get(final int index) {
+		return this.delegate.get(index);
+	}
+
+	@Override
+	public E getElementAt(final int index) {
+		return this.delegate.get(index);
+	}
+
+	@Override
+	public int getSize() {
+		return this.delegate.size();
+	}
+
+	@Override
+	public int indexOf(final Object elem) {
+		return this.delegate.indexOf(elem);
+	}
+
+	@Override
+	public boolean isEmpty() {
+		return this.delegate.isEmpty();
+	}
+
+	@Override
+	public Iterator<E> iterator() {
+		return this.delegate.iterator();
+	}
+
+	@Override
+	public int lastIndexOf(final Object elem) {
+		return this.delegate.lastIndexOf(elem);
+	}
+
+	@Override
+	public ListIterator<E> listIterator() {
+		return this.delegate.listIterator();
+	}
+
+	@Override
+	public ListIterator<E> listIterator(final int index) {
+		return this.delegate.listIterator(index);
+	}
+
+	@Override
+	public E remove(final int index) {
+		final E returnValue = this.delegate.get(index);
+		this.delegate.remove(index);
+		this.fireIntervalRemoved(this, index, index);
+		return returnValue;
+	}
+
+	@Override
+	public boolean remove(final Object o) {
+		final int index = this.delegate.indexOf(o);
+		final boolean returnValue = this.delegate.remove(o);
+		this.fireIntervalRemoved(this, index, index);
+		return returnValue;
+	}
+
+	@Override
+	public boolean removeAll(final Collection<?> c) {
+		boolean changed = false;
+		for (final Object e : c) {
+			if (this.remove(e)) {
+				changed = true;
+			}
+		}
+		return changed;
+	}
+
+	@Override
+	public boolean retainAll(final Collection<?> c) {
+		final int oldSize = this.size();
+		final boolean returnValue = this.delegate.retainAll(c);
+		this.fireIntervalRemoved(this, this.size(), oldSize - 1);
+		return returnValue;
+	}
+
+	@Override
+	public E set(final int index, final E element) {
+		final E returnValue = this.delegate.get(index);
+		this.delegate.set(index, element);
+		this.fireContentsChanged(this, index, index);
+		return returnValue;
+	}
+
+	@Override
+	public int size() {
+		return this.delegate.size();
+	}
+
+	@Override
+	public List<E> subList(final int fromIndex, final int toIndex) {
+		return this.delegate.subList(fromIndex, toIndex);
+	}
+
+	@Override
+	public Object[] toArray() {
+		return this.delegate.toArray();
+	}
+
+	@Override
+	public <T> T[] toArray(final T[] a) {
+		return this.delegate.toArray(a);
+	}
+
+	@Override
+	public String toString() {
+		return this.delegate.toString();
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/FilterComparator.java b/src/main/java/org/unitConverter/converterGUI/FilterComparator.java
new file mode 100644
index 0000000..9b77f21
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/FilterComparator.java
@@ -0,0 +1,129 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.util.Comparator;
+import java.util.Objects;
+
+/**
+ * A comparator that compares strings using a filter.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-15
+ * @since v0.1.0
+ */
+final class FilterComparator implements Comparator<String> {
+	/**
+	 * The filter that the comparator is filtered by.
+	 * 
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	private final String filter;
+	/**
+	 * The comparator to use if the arguments are otherwise equal.
+	 * 
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	private final Comparator<String> comparator;
+	/**
+	 * Whether or not the comparison is case-sensitive.
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	private final boolean caseSensitive;
+
+	/**
+	 * Creates the {@code FilterComparator}.
+	 * 
+	 * @param filter
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	public FilterComparator(final String filter) {
+		this(filter, null);
+	}
+
+	/**
+	 * Creates the {@code FilterComparator}.
+	 * 
+	 * @param filter
+	 *            string to filter by
+	 * @param comparator
+	 *            comparator to fall back to if all else fails, null is compareTo.
+	 * @throws NullPointerException
+	 *             if filter is null
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	public FilterComparator(final String filter, final Comparator<String> comparator) {
+		this(filter, comparator, false);
+	}
+
+	/**
+	 * Creates the {@code FilterComparator}.
+	 * 
+	 * @param filter
+	 *            string to filter by
+	 * @param comparator
+	 *            comparator to fall back to if all else fails, null is compareTo.
+	 * @param caseSensitive
+	 *            whether or not the comparator is case-sensitive
+	 * @throws NullPointerException
+	 *             if filter is null
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	public FilterComparator(final String filter, final Comparator<String> comparator, final boolean caseSensitive) {
+		this.filter = Objects.requireNonNull(filter, "filter must not be null.");
+		this.comparator = comparator;
+		this.caseSensitive = caseSensitive;
+	}
+
+	@Override
+	public int compare(final String arg0, final String arg1) {
+		// if this is case insensitive, make them lowercase
+		final String str0, str1;
+		if (this.caseSensitive) {
+			str0 = arg0;
+			str1 = arg1;
+		} else {
+			str0 = arg0.toLowerCase();
+			str1 = arg1.toLowerCase();
+		}
+
+		// elements that start with the filter always go first
+		if (str0.startsWith(this.filter) && !str1.startsWith(this.filter))
+			return -1;
+		else if (!str0.startsWith(this.filter) && str1.startsWith(this.filter))
+			return 1;
+
+		// elements that contain the filter but don't start with them go next
+		if (str0.contains(this.filter) && !str1.contains(this.filter))
+			return -1;
+		else if (!str0.contains(this.filter) && !str1.contains(this.filter))
+			return 1;
+
+		// other elements go last
+		if (this.comparator == null)
+			return str0.compareTo(str1);
+		else
+			return this.comparator.compare(str0, str1);
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/GridBagBuilder.java b/src/main/java/org/unitConverter/converterGUI/GridBagBuilder.java
new file mode 100644
index 0000000..f1229b2
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/GridBagBuilder.java
@@ -0,0 +1,479 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.awt.GridBagConstraints;
+import java.awt.Insets;
+
+/**
+ * A builder for Java's {@link java.awt.GridBagConstraints} class.
+ * 
+ * @author Adrien Hopkins
+ * @since 2018-11-30
+ * @since v0.1.0
+ */
+final class GridBagBuilder {
+	/**
+	 * The built {@code GridBagConstraints}'s {@code gridx} property.
+	 * <p>
+	 * Specifies the cell containing the leading edge of the component's display area, where the first cell in a row has
+	 * <code>gridx=0</code>. The leading edge of a component's display area is its left edge for a horizontal,
+	 * left-to-right container and its right edge for a horizontal, right-to-left container. The value
+	 * <code>RELATIVE</code> specifies that the component be placed immediately following the component that was added
+	 * to the container just before this component was added.
+	 * <p>
+	 * The default value is <code>RELATIVE</code>. <code>gridx</code> should be a non-negative value.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#gridy
+	 * @see java.awt.ComponentOrientation
+	 */
+	private final int gridx;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code gridy} property.
+	 * <p>
+	 * Specifies the cell at the top of the component's display area, where the topmost cell has <code>gridy=0</code>.
+	 * The value <code>RELATIVE</code> specifies that the component be placed just below the component that was added to
+	 * the container just before this component was added.
+	 * <p>
+	 * The default value is <code>RELATIVE</code>. <code>gridy</code> should be a non-negative value.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#gridx
+	 */
+	private final int gridy;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code gridwidth} property.
+	 * <p>
+	 * Specifies the number of cells in a row for the component's display area.
+	 * <p>
+	 * Use <code>REMAINDER</code> to specify that the component's display area will be from <code>gridx</code> to the
+	 * last cell in the row. Use <code>RELATIVE</code> to specify that the component's display area will be from
+	 * <code>gridx</code> to the next to the last one in its row.
+	 * <p>
+	 * <code>gridwidth</code> should be non-negative and the default value is 1.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#gridheight
+	 */
+	private final int gridwidth;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code gridheight} property.
+	 * <p>
+	 * Specifies the number of cells in a column for the component's display area.
+	 * <p>
+	 * Use <code>REMAINDER</code> to specify that the component's display area will be from <code>gridy</code> to the
+	 * last cell in the column. Use <code>RELATIVE</code> to specify that the component's display area will be from
+	 * <code>gridy</code> to the next to the last one in its column.
+	 * <p>
+	 * <code>gridheight</code> should be a non-negative value and the default value is 1.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#gridwidth
+	 */
+	private final int gridheight;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code weightx} property.
+	 * <p>
+	 * Specifies how to distribute extra horizontal space.
+	 * <p>
+	 * The grid bag layout manager calculates the weight of a column to be the maximum <code>weightx</code> of all the
+	 * components in a column. If the resulting layout is smaller horizontally than the area it needs to fill, the extra
+	 * space is distributed to each column in proportion to its weight. A column that has a weight of zero receives no
+	 * extra space.
+	 * <p>
+	 * If all the weights are zero, all the extra space appears between the grids of the cell and the left and right
+	 * edges.
+	 * <p>
+	 * The default value of this field is <code>0</code>. <code>weightx</code> should be a non-negative value.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#weighty
+	 */
+	private double weightx;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code weighty} property.
+	 * <p>
+	 * Specifies how to distribute extra vertical space.
+	 * <p>
+	 * The grid bag layout manager calculates the weight of a row to be the maximum <code>weighty</code> of all the
+	 * components in a row. If the resulting layout is smaller vertically than the area it needs to fill, the extra
+	 * space is distributed to each row in proportion to its weight. A row that has a weight of zero receives no extra
+	 * space.
+	 * <p>
+	 * If all the weights are zero, all the extra space appears between the grids of the cell and the top and bottom
+	 * edges.
+	 * <p>
+	 * The default value of this field is <code>0</code>. <code>weighty</code> should be a non-negative value.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#weightx
+	 */
+	private double weighty;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code anchor} property.
+	 * <p>
+	 * This field is used when the component is smaller than its display area. It determines where, within the display
+	 * area, to place the component.
+	 * <p>
+	 * There are three kinds of possible values: orientation relative, baseline relative and absolute. Orientation
+	 * relative values are interpreted relative to the container's component orientation property, baseline relative
+	 * values are interpreted relative to the baseline and absolute values are not. The absolute values are:
+	 * <code>CENTER</code>, <code>NORTH</code>, <code>NORTHEAST</code>, <code>EAST</code>, <code>SOUTHEAST</code>,
+	 * <code>SOUTH</code>, <code>SOUTHWEST</code>, <code>WEST</code>, and <code>NORTHWEST</code>. The orientation
+	 * relative values are: <code>PAGE_START</code>, <code>PAGE_END</code>, <code>LINE_START</code>,
+	 * <code>LINE_END</code>, <code>FIRST_LINE_START</code>, <code>FIRST_LINE_END</code>, <code>LAST_LINE_START</code>
+	 * and <code>LAST_LINE_END</code>. The baseline relative values are: <code>BASELINE</code>,
+	 * <code>BASELINE_LEADING</code>, <code>BASELINE_TRAILING</code>, <code>ABOVE_BASELINE</code>,
+	 * <code>ABOVE_BASELINE_LEADING</code>, <code>ABOVE_BASELINE_TRAILING</code>, <code>BELOW_BASELINE</code>,
+	 * <code>BELOW_BASELINE_LEADING</code>, and <code>BELOW_BASELINE_TRAILING</code>. The default value is
+	 * <code>CENTER</code>.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.ComponentOrientation
+	 */
+	private int anchor;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code fill} property.
+	 * <p>
+	 * This field is used when the component's display area is larger than the component's requested size. It determines
+	 * whether to resize the component, and if so, how.
+	 * <p>
+	 * The following values are valid for <code>fill</code>:
+	 *
+	 * <ul>
+	 * <li><code>NONE</code>: Do not resize the component.
+	 * <li><code>HORIZONTAL</code>: Make the component wide enough to fill its display area horizontally, but do not
+	 * change its height.
+	 * <li><code>VERTICAL</code>: Make the component tall enough to fill its display area vertically, but do not change
+	 * its width.
+	 * <li><code>BOTH</code>: Make the component fill its display area entirely.
+	 * </ul>
+	 * <p>
+	 * The default value is <code>NONE</code>.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 */
+	private int fill;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code insets} property.
+	 * <p>
+	 * This field specifies the external padding of the component, the minimum amount of space between the component and
+	 * the edges of its display area.
+	 * <p>
+	 * The default value is <code>new Insets(0, 0, 0, 0)</code>.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 */
+	private Insets insets;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code ipadx} property.
+	 * <p>
+	 * This field specifies the internal padding of the component, how much space to add to the minimum width of the
+	 * component. The width of the component is at least its minimum width plus <code>ipadx</code> pixels.
+	 * <p>
+	 * The default value is <code>0</code>.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#ipady
+	 */
+	private int ipadx;
+
+	/**
+	 * The built {@code GridBagConstraints}'s {@code ipady} property.
+	 * <p>
+	 * This field specifies the internal padding, that is, how much space to add to the minimum height of the component.
+	 * The height of the component is at least its minimum height plus <code>ipady</code> pixels.
+	 * <p>
+	 * The default value is 0.
+	 * 
+	 * @serial
+	 * @see #clone()
+	 * @see java.awt.GridBagConstraints#ipadx
+	 */
+	private int ipady;
+
+	/**
+	 * @param gridx
+	 *            x position
+	 * @param gridy
+	 *            y position
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder(final int gridx, final int gridy) {
+		this(gridx, gridy, 1, 1);
+	}
+
+	/**
+	 * @param gridx
+	 *            x position
+	 * @param gridy
+	 *            y position
+	 * @param gridwidth
+	 *            number of cells occupied horizontally
+	 * @param gridheight
+	 *            number of cells occupied vertically
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder(final int gridx, final int gridy, final int gridwidth, final int gridheight) {
+		this(gridx, gridy, gridwidth, gridheight, 0.0, 0.0, GridBagConstraints.CENTER, GridBagConstraints.NONE,
+				new Insets(0, 0, 0, 0), 0, 0);
+	}
+
+	/**
+	 * @param gridx
+	 *            x position
+	 * @param gridy
+	 *            y position
+	 * @param gridwidth
+	 *            number of cells occupied horizontally
+	 * @param gridheight
+	 *            number of cells occupied vertically
+	 * @param weightx
+	 * @param weighty
+	 * @param anchor
+	 * @param fill
+	 * @param insets
+	 * @param ipadx
+	 * @param ipady
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	private GridBagBuilder(final int gridx, final int gridy, final int gridwidth, final int gridheight,
+			final double weightx, final double weighty, final int anchor, final int fill, final Insets insets,
+			final int ipadx, final int ipady) {
+		super();
+		this.gridx = gridx;
+		this.gridy = gridy;
+		this.gridwidth = gridwidth;
+		this.gridheight = gridheight;
+		this.weightx = weightx;
+		this.weighty = weighty;
+		this.anchor = anchor;
+		this.fill = fill;
+		this.insets = (Insets) insets.clone();
+		this.ipadx = ipadx;
+		this.ipady = ipady;
+	}
+
+	/**
+	 * @return {@code GridBagConstraints} created by this builder
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagConstraints build() {
+		return new GridBagConstraints(this.gridx, this.gridy, this.gridwidth, this.gridheight, this.weightx,
+				this.weighty, this.anchor, this.fill, this.insets, this.ipadx, this.ipady);
+	}
+
+	/**
+	 * @return anchor
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getAnchor() {
+		return this.anchor;
+	}
+
+	/**
+	 * @return fill
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getFill() {
+		return this.fill;
+	}
+
+	/**
+	 * @return gridheight
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getGridheight() {
+		return this.gridheight;
+	}
+
+	/**
+	 * @return gridwidth
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getGridwidth() {
+		return this.gridwidth;
+	}
+
+	/**
+	 * @return gridx
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getGridx() {
+		return this.gridx;
+	}
+
+	/**
+	 * @return gridy
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getGridy() {
+		return this.gridy;
+	}
+
+	/**
+	 * @return insets
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public Insets getInsets() {
+		return this.insets;
+	}
+
+	/**
+	 * @return ipadx
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getIpadx() {
+		return this.ipadx;
+	}
+
+	/**
+	 * @return ipady
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public int getIpady() {
+		return this.ipady;
+	}
+
+	/**
+	 * @return weightx
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public double getWeightx() {
+		return this.weightx;
+	}
+
+	/**
+	 * @return weighty
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public double getWeighty() {
+		return this.weighty;
+	}
+
+	/**
+	 * @param anchor
+	 *            anchor to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setAnchor(final int anchor) {
+		this.anchor = anchor;
+		return this;
+	}
+
+	/**
+	 * @param fill
+	 *            fill to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setFill(final int fill) {
+		this.fill = fill;
+		return this;
+	}
+
+	/**
+	 * @param insets
+	 *            insets to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setInsets(final Insets insets) {
+		this.insets = insets;
+		return this;
+	}
+
+	/**
+	 * @param ipadx
+	 *            ipadx to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setIpadx(final int ipadx) {
+		this.ipadx = ipadx;
+		return this;
+	}
+
+	/**
+	 * @param ipady
+	 *            ipady to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setIpady(final int ipady) {
+		this.ipady = ipady;
+		return this;
+	}
+
+	/**
+	 * @param weightx
+	 *            weightx to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setWeightx(final double weightx) {
+		this.weightx = weightx;
+		return this;
+	}
+
+	/**
+	 * @param weighty
+	 *            weighty to set
+	 * @since 2018-11-30
+	 * @since v0.1.0
+	 */
+	public GridBagBuilder setWeighty(final double weighty) {
+		this.weighty = weighty;
+		return this;
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/MutablePredicate.java b/src/main/java/org/unitConverter/converterGUI/MutablePredicate.java
new file mode 100644
index 0000000..e15b3cd
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/MutablePredicate.java
@@ -0,0 +1,70 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.util.function.Predicate;
+
+/**
+ * A container for a predicate, which can be changed later.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-04-13
+ * @since v0.2.0
+ */
+final class MutablePredicate<T> implements Predicate<T> {
+	/**
+	 * The predicate stored in this {@code MutablePredicate}
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private Predicate<T> predicate;
+
+	/**
+	 * Creates the {@code MutablePredicate}.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public MutablePredicate(final Predicate<T> predicate) {
+		this.predicate = predicate;
+	}
+
+	/**
+	 * @return predicate
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public final Predicate<T> getPredicate() {
+		return this.predicate;
+	}
+
+	/**
+	 * @param predicate
+	 *            new value of predicate
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public final void setPredicate(final Predicate<T> predicate) {
+		this.predicate = predicate;
+	}
+
+	@Override
+	public boolean test(final T t) {
+		return this.predicate.test(t);
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/SearchBoxList.java b/src/main/java/org/unitConverter/converterGUI/SearchBoxList.java
new file mode 100644
index 0000000..10ef589
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/SearchBoxList.java
@@ -0,0 +1,307 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.awt.BorderLayout;
+import java.awt.Color;
+import java.awt.event.FocusEvent;
+import java.awt.event.FocusListener;
+import java.util.ArrayList;
+import java.util.Collection;
+import java.util.Comparator;
+import java.util.function.Predicate;
+
+import javax.swing.JList;
+import javax.swing.JPanel;
+import javax.swing.JScrollPane;
+import javax.swing.JTextField;
+
+/**
+ * @author Adrien Hopkins
+ * @since 2019-04-13
+ * @since v0.2.0
+ */
+final class SearchBoxList extends JPanel {
+	
+	/**
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private static final long serialVersionUID = 6226930279415983433L;
+	
+	/**
+	 * The text to place in an empty search box.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private static final String EMPTY_TEXT = "Search...";
+	
+	/**
+	 * The color to use for an empty foreground.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private static final Color EMPTY_FOREGROUND = new Color(192, 192, 192);
+	
+	// the components
+	private final Collection<String> itemsToFilter;
+	private final DelegateListModel<String> listModel;
+	private final JTextField searchBox;
+	private final JList<String> searchItems;
+	
+	private boolean searchBoxEmpty = true;
+	
+	// I need to do this because, for some reason, Swing is auto-focusing my
+	// search box without triggering a focus
+	// event.
+	private boolean searchBoxFocused = false;
+	
+	private Predicate<String> customSearchFilter = o -> true;
+	private final Comparator<String> defaultOrdering;
+	private final boolean caseSensitive;
+	
+	/**
+	 * Creates the {@code SearchBoxList}.
+	 * 
+	 * @param itemsToFilter items to put in the list
+	 * @since 2019-04-14
+	 */
+	public SearchBoxList(final Collection<String> itemsToFilter) {
+		this(itemsToFilter, null, false);
+	}
+	
+	/**
+	 * Creates the {@code SearchBoxList}.
+	 * 
+	 * @param itemsToFilter   items to put in the list
+	 * @param defaultOrdering default ordering of items after filtration
+	 *                        (null=Comparable)
+	 * @param caseSensitive   whether or not the filtration is case-sensitive
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public SearchBoxList(final Collection<String> itemsToFilter,
+			final Comparator<String> defaultOrdering,
+			final boolean caseSensitive) {
+		super(new BorderLayout(), true);
+		this.itemsToFilter = itemsToFilter;
+		this.defaultOrdering = defaultOrdering;
+		this.caseSensitive = caseSensitive;
+		
+		// create the components
+		this.listModel = new DelegateListModel<>(new ArrayList<>(itemsToFilter));
+		this.searchItems = new JList<>(this.listModel);
+		
+		this.searchBox = new JTextField(EMPTY_TEXT);
+		this.searchBox.setForeground(EMPTY_FOREGROUND);
+		
+		// add them to the panel
+		this.add(this.searchBox, BorderLayout.PAGE_START);
+		this.add(new JScrollPane(this.searchItems), BorderLayout.CENTER);
+		
+		// set up the search box
+		this.searchBox.addFocusListener(new FocusListener() {
+			@Override
+			public void focusGained(final FocusEvent e) {
+				SearchBoxList.this.searchBoxFocusGained(e);
+			}
+			
+			@Override
+			public void focusLost(final FocusEvent e) {
+				SearchBoxList.this.searchBoxFocusLost(e);
+			}
+		});
+		
+		this.searchBox.addCaretListener(e -> this.searchBoxTextChanged());
+		this.searchBoxEmpty = true;
+	}
+	
+	/**
+	 * Adds an additional filter for searching.
+	 * 
+	 * @param filter filter to add.
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public void addSearchFilter(final Predicate<String> filter) {
+		this.customSearchFilter = this.customSearchFilter.and(filter);
+	}
+	
+	/**
+	 * Resets the search filter.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public void clearSearchFilters() {
+		this.customSearchFilter = o -> true;
+	}
+	
+	/**
+	 * @return this component's search box component
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	public final JTextField getSearchBox() {
+		return this.searchBox;
+	}
+	
+	/**
+	 * @param searchText text to search for
+	 * @return a filter that filters out that text, based on this list's case
+	 *         sensitive setting
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	private Predicate<String> getSearchFilter(final String searchText) {
+		if (this.caseSensitive)
+			return string -> string.contains(searchText);
+		else
+			return string -> string.toLowerCase()
+					.contains(searchText.toLowerCase());
+	}
+	
+	/**
+	 * @return this component's list component
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	public final JList<String> getSearchList() {
+		return this.searchItems;
+	}
+	
+	/**
+	 * @return index selected in item list
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	public int getSelectedIndex() {
+		return this.searchItems.getSelectedIndex();
+	}
+	
+	/**
+	 * @return value selected in item list
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public String getSelectedValue() {
+		return this.searchItems.getSelectedValue();
+	}
+	
+	/**
+	 * Re-applies the filters.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public void reapplyFilter() {
+		final String searchText = this.searchBoxEmpty ? ""
+				: this.searchBox.getText();
+		final FilterComparator comparator = new FilterComparator(searchText,
+				this.defaultOrdering, this.caseSensitive);
+		final Predicate<String> searchFilter = this.getSearchFilter(searchText);
+		
+		this.listModel.clear();
+		this.itemsToFilter.forEach(string -> {
+			if (searchFilter.test(string)) {
+				this.listModel.add(string);
+			}
+		});
+		
+		// applies the custom filters
+		this.listModel.removeIf(this.customSearchFilter.negate());
+		
+		// sorts the remaining items
+		this.listModel.sort(comparator);
+	}
+	
+	/**
+	 * Runs whenever the search box gains focus.
+	 * 
+	 * @param e focus event
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private void searchBoxFocusGained(final FocusEvent e) {
+		this.searchBoxFocused = true;
+		if (this.searchBoxEmpty) {
+			this.searchBox.setText("");
+			this.searchBox.setForeground(Color.BLACK);
+		}
+	}
+	
+	/**
+	 * Runs whenever the search box loses focus.
+	 * 
+	 * @param e focus event
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private void searchBoxFocusLost(final FocusEvent e) {
+		this.searchBoxFocused = false;
+		if (this.searchBoxEmpty) {
+			this.searchBox.setText(EMPTY_TEXT);
+			this.searchBox.setForeground(EMPTY_FOREGROUND);
+		}
+	}
+	
+	/**
+	 * Runs whenever the text in the search box is changed.
+	 * <p>
+	 * Reapplies the search filter, and custom filters.
+	 * </p>
+	 * 
+	 * @since 2019-04-14
+	 * @since v0.2.0
+	 */
+	private void searchBoxTextChanged() {
+		if (this.searchBoxFocused) {
+			this.searchBoxEmpty = this.searchBox.getText().equals("");
+		}
+		final String searchText = this.searchBoxEmpty ? ""
+				: this.searchBox.getText();
+		final FilterComparator comparator = new FilterComparator(searchText,
+				this.defaultOrdering, this.caseSensitive);
+		final Predicate<String> searchFilter = this.getSearchFilter(searchText);
+		
+		// initialize list with items that match the filter then sort
+		this.listModel.clear();
+		this.itemsToFilter.forEach(string -> {
+			if (searchFilter.test(string)) {
+				this.listModel.add(string);
+			}
+		});
+		
+		// applies the custom filters
+		this.listModel.removeIf(this.customSearchFilter.negate());
+		
+		// sorts the remaining items
+		this.listModel.sort(comparator);
+	}
+	
+	/**
+	 * Manually updates the search box's item list.
+	 * 
+	 * @since 2020-08-27
+	 */
+	public void updateList() {
+		this.searchBoxTextChanged();
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/UnitConverterGUI.java b/src/main/java/org/unitConverter/converterGUI/UnitConverterGUI.java
new file mode 100644
index 0000000..ee1bcc3
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/UnitConverterGUI.java
@@ -0,0 +1,1477 @@
+/**
+ * Copyright (C) 2018-2021 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.converterGUI;
+
+import java.awt.BorderLayout;
+import java.awt.GridBagConstraints;
+import java.awt.GridBagLayout;
+import java.awt.GridLayout;
+import java.awt.event.KeyEvent;
+import java.io.BufferedWriter;
+import java.io.IOException;
+import java.io.InputStream;
+import java.math.BigDecimal;
+import java.math.MathContext;
+import java.math.RoundingMode;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.text.DecimalFormat;
+import java.text.NumberFormat;
+import java.text.ParseException;
+import java.util.ArrayList;
+import java.util.Comparator;
+import java.util.HashSet;
+import java.util.List;
+import java.util.NoSuchElementException;
+import java.util.Scanner;
+import java.util.Set;
+import java.util.function.Predicate;
+import java.util.stream.Collectors;
+
+import javax.swing.BorderFactory;
+import javax.swing.BoxLayout;
+import javax.swing.ButtonGroup;
+import javax.swing.JButton;
+import javax.swing.JCheckBox;
+import javax.swing.JComboBox;
+import javax.swing.JFormattedTextField;
+import javax.swing.JFrame;
+import javax.swing.JLabel;
+import javax.swing.JOptionPane;
+import javax.swing.JPanel;
+import javax.swing.JRadioButton;
+import javax.swing.JScrollPane;
+import javax.swing.JSlider;
+import javax.swing.JTabbedPane;
+import javax.swing.JTextArea;
+import javax.swing.JTextField;
+import javax.swing.UIManager;
+import javax.swing.UnsupportedLookAndFeelException;
+import javax.swing.WindowConstants;
+import javax.swing.border.TitledBorder;
+
+import org.unitConverter.math.ConditionalExistenceCollections;
+import org.unitConverter.math.ObjectProduct;
+import org.unitConverter.unit.BaseDimension;
+import org.unitConverter.unit.BritishImperial;
+import org.unitConverter.unit.LinearUnit;
+import org.unitConverter.unit.LinearUnitValue;
+import org.unitConverter.unit.NameSymbol;
+import org.unitConverter.unit.SI;
+import org.unitConverter.unit.Unit;
+import org.unitConverter.unit.UnitDatabase;
+import org.unitConverter.unit.UnitPrefix;
+import org.unitConverter.unit.UnitValue;
+
+/**
+ * @author Adrien Hopkins
+ * @since 2018-12-27
+ * @since v0.1.0
+ */
+final class UnitConverterGUI {
+	/**
+	 * A tab in the View.
+	 */
+	private enum Pane {
+		UNIT_CONVERTER, EXPRESSION_CONVERTER, UNIT_VIEWER, PREFIX_VIEWER, ABOUT,
+		SETTINGS;
+	}
+	
+	private static class Presenter {
+		/** The default place where settings are stored. */
+		private static final String DEFAULT_SETTINGS_FILEPATH = "settings.txt";
+		/** The default place where units are stored. */
+		private static final String DEFAULT_UNITS_FILEPATH = "/unitsfile.txt";
+		/** The default place where dimensions are stored. */
+		private static final String DEFAULT_DIMENSIONS_FILEPATH = "/dimensionfile.txt";
+		/** The default place where exceptions are stored. */
+		private static final String DEFAULT_EXCEPTIONS_FILEPATH = "/metric_exceptions.txt";
+		
+		/**
+		 * Adds default units and dimensions to a database.
+		 * 
+		 * @param database database to add to
+		 * @since 2019-04-14
+		 * @since v0.2.0
+		 */
+		private static void addDefaults(final UnitDatabase database) {
+			database.addUnit("metre", SI.METRE);
+			database.addUnit("kilogram", SI.KILOGRAM);
+			database.addUnit("gram", SI.KILOGRAM.dividedBy(1000));
+			database.addUnit("second", SI.SECOND);
+			database.addUnit("ampere", SI.AMPERE);
+			database.addUnit("kelvin", SI.KELVIN);
+			database.addUnit("mole", SI.MOLE);
+			database.addUnit("candela", SI.CANDELA);
+			database.addUnit("bit", SI.BIT);
+			database.addUnit("unit", SI.ONE);
+			// nonlinear units - must be loaded manually
+			database.addUnit("tempCelsius", SI.CELSIUS);
+			database.addUnit("tempFahrenheit", BritishImperial.FAHRENHEIT);
+			
+			// load initial dimensions
+			database.addDimension("LENGTH", SI.Dimensions.LENGTH);
+			database.addDimension("MASS", SI.Dimensions.MASS);
+			database.addDimension("TIME", SI.Dimensions.TIME);
+			database.addDimension("TEMPERATURE", SI.Dimensions.TEMPERATURE);
+		}
+		
+		/**
+		 * Gets the text of a resource file as a set of strings (each one is one
+		 * line of the text).
+		 *
+		 * @param filename filename to get resource from
+		 * @return contents of file
+		 * @since 2021-03-27
+		 */
+		public static final List<String> getLinesFromResource(String filename) {
+			final List<String> lines = new ArrayList<>();
+			
+			try (InputStream stream = inputStream(filename);
+					Scanner scanner = new Scanner(stream)) {
+				while (scanner.hasNextLine()) {
+					lines.add(scanner.nextLine());
+				}
+			} catch (final IOException e) {
+				throw new AssertionError(
+						"Error occurred while loading file " + filename, e);
+			}
+			
+			return lines;
+		}
+		
+		/**
+		 * Gets an input stream for a resource file.
+		 *
+		 * @param filepath file to use as resource
+		 * @return obtained Path
+		 * @since 2021-03-27
+		 */
+		private static final InputStream inputStream(String filepath) {
+			return UnitConverterGUI.class.getResourceAsStream(filepath);
+		}
+		
+		/**
+		 * @return {@code line} with any comments removed.
+		 * @since 2021-03-13
+		 */
+		private static final String withoutComments(String line) {
+			final int index = line.indexOf('#');
+			return index == -1 ? line : line.substring(index);
+		}
+		
+		/** The presenter's associated view. */
+		private final View view;
+		
+		/** The units known by the program. */
+		private final UnitDatabase database;
+		
+		/** The names of all of the units */
+		private final List<String> unitNames;
+		
+		/** The names of all of the prefixes */
+		private final List<String> prefixNames;
+		
+		/** The names of all of the dimensions */
+		private final List<String> dimensionNames;
+		
+		/** Unit names that are ignored by the metric-only/imperial-only filter */
+		private final Set<String> metricExceptions;
+		
+		private final Comparator<String> prefixNameComparator;
+		
+		/** A boolean remembering whether or not one-way conversion is on */
+		private boolean oneWay = true;
+		/** The prefix rule */
+		private DefaultPrefixRepetitionRule prefixRule = null;
+		
+		// conditions for existence of From and To entries
+		// used for one-way conversion
+		private final MutablePredicate<String> fromExistenceCondition = new MutablePredicate<>(
+				s -> true);
+		
+		private final MutablePredicate<String> toExistenceCondition = new MutablePredicate<>(
+				s -> true);
+		
+		/*
+		 * Rounding-related settings. I am using my own system, and not
+		 * MathContext, because MathContext does not support decimal place based
+		 * or scientific rounding, only significant digit based rounding.
+		 */
+		private int precision = 6;
+		
+		private RoundingType roundingType = RoundingType.SIGNIFICANT_DIGITS;
+		
+		/**
+		 * Creates the presenter.
+		 * 
+		 * @param view presenter's associated view
+		 * @since 2018-12-27
+		 * @since v0.1.0
+		 */
+		Presenter(final View view) {
+			this.view = view;
+			
+			// load initial units
+			this.database = new UnitDatabase(
+					DefaultPrefixRepetitionRule.NO_RESTRICTION);
+			Presenter.addDefaults(this.database);
+			
+			// load units and prefixes
+			try (final InputStream units = inputStream(DEFAULT_UNITS_FILEPATH)) {
+				this.database.loadUnitsFromStream(units);
+			} catch (final IOException e) {
+				throw new AssertionError("Loading of unitsfile.txt failed.", e);
+			}
+			
+			// load dimensions
+			try (final InputStream dimensions = inputStream(
+					DEFAULT_DIMENSIONS_FILEPATH)) {
+				this.database.loadDimensionsFromStream(dimensions);
+			} catch (final IOException e) {
+				throw new AssertionError("Loading of dimensionfile.txt failed.", e);
+			}
+			
+			// load metric exceptions
+			try {
+				this.metricExceptions = new HashSet<>();
+				try (InputStream exceptions = inputStream(
+						DEFAULT_EXCEPTIONS_FILEPATH);
+						Scanner scanner = new Scanner(exceptions)) {
+					while (scanner.hasNextLine()) {
+						final String line = Presenter
+								.withoutComments(scanner.nextLine());
+						if (!line.isBlank()) {
+							this.metricExceptions.add(line);
+						}
+					}
+				}
+			} catch (final IOException e) {
+				throw new AssertionError("Loading of metric_exceptions.txt failed.",
+						e);
+			}
+			
+			// load settings - requires database to exist
+			if (Files.exists(this.getSettingsFile())) {
+				this.loadSettings();
+			}
+			
+			// a comparator that can be used to compare prefix names
+			// any name that does not exist is less than a name that does.
+			// otherwise, they are compared by value
+			this.prefixNameComparator = (o1, o2) -> {
+				if (!Presenter.this.database.containsPrefixName(o1))
+					return -1;
+				else if (!Presenter.this.database.containsPrefixName(o2))
+					return 1;
+				
+				final UnitPrefix p1 = Presenter.this.database.getPrefix(o1);
+				final UnitPrefix p2 = Presenter.this.database.getPrefix(o2);
+				
+				if (p1.getMultiplier() < p2.getMultiplier())
+					return -1;
+				else if (p1.getMultiplier() > p2.getMultiplier())
+					return 1;
+				
+				return o1.compareTo(o2);
+			};
+			
+			this.unitNames = new ArrayList<>(
+					this.database.unitMapPrefixless().keySet());
+			this.unitNames.sort(null); // sorts it using Comparable
+			
+			this.prefixNames = new ArrayList<>(this.database.prefixMap().keySet());
+			this.prefixNames.sort(this.prefixNameComparator); // sorts it using my
+																				// comparator
+			
+			this.dimensionNames = new DelegateListModel<>(
+					new ArrayList<>(this.database.dimensionMap().keySet()));
+			this.dimensionNames.sort(null); // sorts it using Comparable
+			
+			// a Predicate that returns true iff the argument is a full base unit
+			final Predicate<Unit> isFullBase = unit -> unit instanceof LinearUnit
+					&& ((LinearUnit) unit).isBase();
+			
+			// print out unit counts
+			System.out.printf(
+					"Successfully loaded %d units with %d unit names (%d base units).%n",
+					new HashSet<>(this.database.unitMapPrefixless().values()).size(),
+					this.database.unitMapPrefixless().size(),
+					new HashSet<>(this.database.unitMapPrefixless().values())
+							.stream().filter(isFullBase).count());
+		}
+		
+		/**
+		 * Converts in the dimension-based converter
+		 * 
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public final void convertDimensionBased() {
+			final String fromSelection = this.view.getFromSelection();
+			if (fromSelection == null) {
+				this.view.showErrorDialog("Error",
+						"No unit selected in From field");
+				return;
+			}
+			final String toSelection = this.view.getToSelection();
+			if (toSelection == null) {
+				this.view.showErrorDialog("Error", "No unit selected in To field");
+				return;
+			}
+			
+			final Unit from = this.database.getUnit(fromSelection);
+			final Unit to = this.database.getUnit(toSelection)
+					.withName(NameSymbol.ofName(toSelection));
+			
+			final UnitValue beforeValue;
+			try {
+				beforeValue = UnitValue.of(from,
+						this.view.getDimensionConverterInput());
+			} catch (final ParseException e) {
+				this.view.showErrorDialog("Error",
+						"Error in parsing: " + e.getMessage());
+				return;
+			}
+			final UnitValue value = beforeValue.convertTo(to);
+			
+			final String output = this.getRoundedString(value);
+			
+			this.view.setDimensionConverterOutputText(
+					String.format("%s = %s", beforeValue, output));
+		}
+		
+		/**
+		 * Runs whenever the convert button is pressed.
+		 * 
+		 * <p>
+		 * Reads and parses a unit expression from the from and to boxes, then
+		 * converts {@code from} to {@code to}. Any errors are shown in
+		 * JOptionPanes.
+		 * </p>
+		 * 
+		 * @since 2019-01-26
+		 * @since v0.1.0
+		 */
+		public final void convertExpressions() {
+			final String fromUnitString = this.view.getFromText();
+			final String toUnitString = this.view.getToText();
+			
+			if (fromUnitString.isEmpty()) {
+				this.view.showErrorDialog("Parse Error",
+						"Please enter a unit expression in the From: box.");
+				return;
+			}
+			if (toUnitString.isEmpty()) {
+				this.view.showErrorDialog("Parse Error",
+						"Please enter a unit expression in the To: box.");
+				return;
+			}
+			
+			final LinearUnitValue from;
+			final Unit to;
+			try {
+				from = this.database.evaluateUnitExpression(fromUnitString);
+			} catch (final IllegalArgumentException | NoSuchElementException e) {
+				this.view.showErrorDialog("Parse Error",
+						"Could not recognize text in From entry: " + e.getMessage());
+				return;
+			}
+			try {
+				to = this.database.getUnitFromExpression(toUnitString);
+			} catch (final IllegalArgumentException | NoSuchElementException e) {
+				this.view.showErrorDialog("Parse Error",
+						"Could not recognize text in To entry: " + e.getMessage());
+				return;
+			}
+			
+			if (to instanceof LinearUnit) {
+				// convert to LinearUnitValue
+				final LinearUnitValue from2;
+				final LinearUnit to2 = ((LinearUnit) to)
+						.withName(NameSymbol.ofName(toUnitString));
+				final boolean useSlash;
+				
+				if (from.canConvertTo(to2)) {
+					from2 = from;
+					useSlash = false;
+				} else if (LinearUnitValue.ONE.dividedBy(from).canConvertTo(to2)) {
+					from2 = LinearUnitValue.ONE.dividedBy(from);
+					useSlash = true;
+				} else {
+					// if I can't convert, leave
+					this.view.showErrorDialog("Conversion Error",
+							String.format("Cannot convert between %s and %s",
+									fromUnitString, toUnitString));
+					return;
+				}
+				
+				final LinearUnitValue converted = from2.convertTo(to2);
+				this.view.setExpressionConverterOutputText((useSlash ? "1 / " : "")
+						+ String.format("%s = %s", fromUnitString,
+								this.getRoundedString(converted, false)));
+				return;
+			} else {
+				// convert to UnitValue
+				final UnitValue from2 = from.asUnitValue();
+				if (from2.canConvertTo(to)) {
+					final UnitValue converted = from2.convertTo(to);
+					
+					this.view
+							.setExpressionConverterOutputText(String.format("%s = %s",
+									fromUnitString, this.getRoundedString(converted)));
+				} else {
+					// if I can't convert, leave
+					this.view.showErrorDialog("Conversion Error",
+							String.format("Cannot convert between %s and %s",
+									fromUnitString, toUnitString));
+				}
+			}
+		}
+		
+		/**
+		 * @return a list of all of the unit dimensions
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public final List<String> dimensionNameList() {
+			return this.dimensionNames;
+		}
+		
+		/**
+		 * @return a list of all the entries in the dimension-based converter's
+		 *         From box
+		 * @since 2020-08-27
+		 */
+		public final Set<String> fromEntries() {
+			return ConditionalExistenceCollections.conditionalExistenceSet(
+					this.unitNameSet(), this.fromExistenceCondition);
+		}
+		
+		/**
+		 * @return a comparator to compare prefix names
+		 * @since 2019-04-14
+		 * @since v0.2.0
+		 */
+		public final Comparator<String> getPrefixNameComparator() {
+			return this.prefixNameComparator;
+		}
+		
+		/**
+		 * Like {@link LinearUnitValue#toString(boolean)}, but obeys this unit
+		 * converter's rounding settings.
+		 * 
+		 * @since 2020-08-04
+		 */
+		private final String getRoundedString(final LinearUnitValue value,
+				boolean showUncertainty) {
+			switch (this.roundingType) {
+			case DECIMAL_PLACES:
+			case SIGNIFICANT_DIGITS:
+				return this.getRoundedString(value.asUnitValue());
+			case SCIENTIFIC:
+				return value.toString(showUncertainty);
+			default:
+				throw new AssertionError("Invalid switch condition.");
+			}
+		}
+		
+		/**
+		 * Like {@link UnitValue#toString()}, but obeys this unit converter's
+		 * rounding settings.
+		 * 
+		 * @since 2020-08-04
+		 */
+		private final String getRoundedString(final UnitValue value) {
+			final BigDecimal unrounded = new BigDecimal(value.getValue());
+			final BigDecimal rounded;
+			int precision = this.precision;
+			
+			switch (this.roundingType) {
+			case DECIMAL_PLACES:
+				rounded = unrounded.setScale(precision, RoundingMode.HALF_EVEN);
+				break;
+			case SCIENTIFIC:
+				precision = 12;
+				//$FALL-THROUGH$
+			case SIGNIFICANT_DIGITS:
+				rounded = unrounded
+						.round(new MathContext(precision, RoundingMode.HALF_EVEN));
+				break;
+			default:
+				throw new AssertionError("Invalid switch condition.");
+			}
+			
+			String output = rounded.toString();
+			
+			// remove trailing zeroes
+			if (output.contains(".")) {
+				while (output.endsWith("0")) {
+					output = output.substring(0, output.length() - 1);
+				}
+				if (output.endsWith(".")) {
+					output = output.substring(0, output.length() - 1);
+				}
+			}
+			
+			return output + " " + value.getUnit().getPrimaryName().get();
+		}
+		
+		/**
+		 * @return The file where settings are stored;
+		 * @since 2020-12-11
+		 */
+		private final Path getSettingsFile() {
+			return Path.of(DEFAULT_SETTINGS_FILEPATH);
+		}
+		
+		/**
+		 * Loads settings from the settings file.
+		 * 
+		 * @since 2021-02-17
+		 */
+		public final void loadSettings() {
+			try {
+				// read file line by line
+				final int lineNum = 0;
+				for (final String line : Files
+						.readAllLines(this.getSettingsFile())) {
+					final int equalsIndex = line.indexOf('=');
+					if (equalsIndex == -1)
+						throw new IllegalStateException(
+								"Settings file is malformed at line " + lineNum);
+					
+					final String param = line.substring(0, equalsIndex);
+					final String value = line.substring(equalsIndex + 1);
+					
+					switch (param) {
+					// set manually to avoid the unnecessary saving of the non-manual
+					// methods
+					case "precision":
+						this.precision = Integer.valueOf(value);
+						break;
+					case "rounding_type":
+						this.roundingType = RoundingType.valueOf(value);
+						break;
+					case "prefix_rule":
+						this.prefixRule = DefaultPrefixRepetitionRule.valueOf(value);
+						this.database.setPrefixRepetitionRule(this.prefixRule);
+						break;
+					case "one_way":
+						this.oneWay = Boolean.valueOf(value);
+						if (this.oneWay) {
+							this.fromExistenceCondition.setPredicate(
+									unitName -> this.metricExceptions.contains(unitName)
+											|| !this.database.getUnit(unitName)
+													.isMetric());
+							this.toExistenceCondition.setPredicate(
+									unitName -> this.metricExceptions.contains(unitName)
+											|| this.database.getUnit(unitName).isMetric());
+						} else {
+							this.fromExistenceCondition.setPredicate(unitName -> true);
+							this.toExistenceCondition.setPredicate(unitName -> true);
+						}
+						break;
+					default:
+						System.err.printf("Warning: unrecognized setting \"%s\".",
+								param);
+						break;
+					}
+				}
+			} catch (final IOException e) {}
+		}
+		
+		/**
+		 * @return a set of all prefix names in the database
+		 * @since 2019-04-14
+		 * @since v0.2.0
+		 */
+		public final Set<String> prefixNameSet() {
+			return this.database.prefixMap().keySet();
+		}
+		
+		/**
+		 * Runs whenever a prefix is selected in the viewer.
+		 * <p>
+		 * Shows its information in the text box to the right.
+		 * </p>
+		 * 
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public final void prefixSelected() {
+			final String prefixName = this.view.getPrefixViewerSelection();
+			if (prefixName == null)
+				return;
+			else {
+				final UnitPrefix prefix = this.database.getPrefix(prefixName);
+				
+				this.view.setPrefixTextBoxText(String.format("%s%nMultiplier: %s",
+						prefixName, prefix.getMultiplier()));
+			}
+		}
+		
+		/**
+		 * Saves the settings to the settings file.
+		 *
+		 * @since 2021-02-17
+		 */
+		public final void saveSettings() {
+			try (BufferedWriter writer = Files
+					.newBufferedWriter(this.getSettingsFile())) {
+				writer.write(String.format("precision=%d\n", this.precision));
+				writer.write(
+						String.format("rounding_type=%s\n", this.roundingType));
+				writer.write(String.format("prefix_rule=%s\n", this.prefixRule));
+				writer.write(String.format("one_way=%s\n", this.oneWay));
+			} catch (final IOException e) {
+				e.printStackTrace();
+				this.view.showErrorDialog("I/O Error",
+						"Error occurred while saving settings: "
+								+ e.getLocalizedMessage());
+			}
+		}
+		
+		/**
+		 * Enables or disables one-way conversion.
+		 * 
+		 * @param oneWay whether one-way conversion should be on (true) or off
+		 *               (false)
+		 * @since 2020-08-27
+		 */
+		public final void setOneWay(boolean oneWay) {
+			this.oneWay = oneWay;
+			if (oneWay) {
+				this.fromExistenceCondition.setPredicate(
+						unitName -> this.metricExceptions.contains(unitName)
+								|| !this.database.getUnit(unitName).isMetric());
+				this.toExistenceCondition.setPredicate(
+						unitName -> this.metricExceptions.contains(unitName)
+								|| this.database.getUnit(unitName).isMetric());
+			} else {
+				this.fromExistenceCondition.setPredicate(unitName -> true);
+				this.toExistenceCondition.setPredicate(unitName -> true);
+			}
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @param precision new value of precision
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public final void setPrecision(final int precision) {
+			this.precision = precision;
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @param prefixRepetitionRule the prefixRepetitionRule to set
+		 * @since 2020-08-26
+		 */
+		public void setPrefixRepetitionRule(
+				Predicate<List<UnitPrefix>> prefixRepetitionRule) {
+			if (prefixRepetitionRule instanceof DefaultPrefixRepetitionRule) {
+				this.prefixRule = (DefaultPrefixRepetitionRule) prefixRepetitionRule;
+			} else {
+				this.prefixRule = null;
+			}
+			this.database.setPrefixRepetitionRule(prefixRepetitionRule);
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @param roundingType the roundingType to set
+		 * @since 2020-07-16
+		 */
+		public final void setRoundingType(RoundingType roundingType) {
+			this.roundingType = roundingType;
+			
+			this.saveSettings();
+		}
+		
+		/**
+		 * @return a list of all the entries in the dimension-based converter's To
+		 *         box
+		 * @since 2020-08-27
+		 */
+		public final Set<String> toEntries() {
+			return ConditionalExistenceCollections.conditionalExistenceSet(
+					this.unitNameSet(), this.toExistenceCondition);
+		}
+		
+		/**
+		 * Returns true if and only if the unit represented by {@code unitName}
+		 * has the dimension represented by {@code dimensionName}.
+		 * 
+		 * @param unitName      name of unit to test
+		 * @param dimensionName name of dimension to test
+		 * @return whether unit has dimenision
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public final boolean unitMatchesDimension(final String unitName,
+				final String dimensionName) {
+			final Unit unit = this.database.getUnit(unitName);
+			final ObjectProduct<BaseDimension> dimension = this.database
+					.getDimension(dimensionName);
+			return unit.getDimension().equals(dimension);
+		}
+		
+		/**
+		 * Runs whenever a unit is selected in the viewer.
+		 * <p>
+		 * Shows its information in the text box to the right.
+		 * </p>
+		 * 
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public final void unitNameSelected() {
+			final String unitName = this.view.getUnitViewerSelection();
+			if (unitName == null)
+				return;
+			else {
+				final Unit unit = this.database.getUnit(unitName);
+				
+				this.view.setUnitTextBoxText(unit.toString());
+			}
+		}
+		
+		/**
+		 * @return a set of all of the unit names
+		 * @since 2019-04-14
+		 * @since v0.2.0
+		 */
+		private final Set<String> unitNameSet() {
+			return this.database.unitMapPrefixless().keySet();
+		}
+	}
+	
+	/**
+	 * Different types of rounding.
+	 * 
+	 * Significant digits: Rounds to a number of digits. i.e. with precision 5,
+	 * 12345.6789 rounds to 12346. Decimal places: Rounds to a number of digits
+	 * after the decimal point, i.e. with precision 5, 12345.6789 rounds to
+	 * 12345.67890. Scientific: Rounds based on the number of digits and
+	 * operations, following standard scientific rounding.
+	 */
+	private static enum RoundingType {
+		SIGNIFICANT_DIGITS, DECIMAL_PLACES, SCIENTIFIC;
+	}
+	
+	private static class View {
+		private static final NumberFormat NUMBER_FORMATTER = new DecimalFormat();
+		
+		/** The view's frame. */
+		private final JFrame frame;
+		/** The view's associated presenter. */
+		private final Presenter presenter;
+		/** The master pane containing all of the tabs. */
+		private final JTabbedPane masterPane;
+		
+		// DIMENSION-BASED CONVERTER
+		/** The panel for inputting values in the dimension-based converter */
+		private final JTextField valueInput;
+		/** The panel for "From" in the dimension-based converter */
+		private final SearchBoxList fromSearch;
+		/** The panel for "To" in the dimension-based converter */
+		private final SearchBoxList toSearch;
+		/** The output area in the dimension-based converter */
+		private final JTextArea dimensionBasedOutput;
+		
+		// EXPRESSION-BASED CONVERTER
+		/** The "From" entry in the conversion panel */
+		private final JTextField fromEntry;
+		/** The "To" entry in the conversion panel */
+		private final JTextField toEntry;
+		/** The output area in the conversion panel */
+		private final JTextArea output;
+		
+		// UNIT AND PREFIX VIEWERS
+		/** The searchable list of unit names in the unit viewer */
+		private final SearchBoxList unitNameList;
+		/** The searchable list of prefix names in the prefix viewer */
+		private final SearchBoxList prefixNameList;
+		/** The text box for unit data in the unit viewer */
+		private final JTextArea unitTextBox;
+		/** The text box for prefix data in the prefix viewer */
+		private final JTextArea prefixTextBox;
+		
+		/**
+		 * Creates the {@code View}.
+		 * 
+		 * @since 2019-01-14
+		 * @since v0.1.0
+		 */
+		public View() {
+			this.presenter = new Presenter(this);
+			this.frame = new JFrame("Unit Converter");
+			this.frame.setDefaultCloseOperation(WindowConstants.EXIT_ON_CLOSE);
+			
+			// enable system look and feel
+			try {
+				UIManager.setLookAndFeel(UIManager.getSystemLookAndFeelClassName());
+			} catch (ClassNotFoundException | InstantiationException
+					| IllegalAccessException | UnsupportedLookAndFeelException e) {
+				// oh well, just use default theme
+				System.err.println("Failed to enable system look-and-feel.");
+				e.printStackTrace();
+			}
+			
+			// create the components
+			this.masterPane = new JTabbedPane();
+			this.unitNameList = new SearchBoxList(this.presenter.unitNameSet());
+			this.prefixNameList = new SearchBoxList(this.presenter.prefixNameSet(),
+					this.presenter.getPrefixNameComparator(), true);
+			this.unitTextBox = new JTextArea();
+			this.prefixTextBox = new JTextArea();
+			this.fromSearch = new SearchBoxList(this.presenter.fromEntries());
+			this.toSearch = new SearchBoxList(this.presenter.toEntries());
+			this.valueInput = new JFormattedTextField(NUMBER_FORMATTER);
+			this.dimensionBasedOutput = new JTextArea(2, 32);
+			this.fromEntry = new JTextField();
+			this.toEntry = new JTextField();
+			this.output = new JTextArea(2, 32);
+			
+			// create more components
+			this.initComponents();
+			
+			this.frame.pack();
+		}
+		
+		/**
+		 * @return the currently selected pane.
+		 * @throws AssertionError if no pane (or an invalid pane) is selected
+		 */
+		public Pane getActivePane() {
+			switch (this.masterPane.getSelectedIndex()) {
+			case 0:
+				return Pane.UNIT_CONVERTER;
+			case 1:
+				return Pane.EXPRESSION_CONVERTER;
+			case 2:
+				return Pane.UNIT_VIEWER;
+			case 3:
+				return Pane.PREFIX_VIEWER;
+			case 4:
+				return Pane.ABOUT;
+			case 5:
+				return Pane.SETTINGS;
+			default:
+				throw new AssertionError("No selected pane, or invalid pane.");
+			}
+		}
+		
+		/**
+		 * @return value in dimension-based converter
+		 * @throws ParseException
+		 * @since 2020-07-07
+		 */
+		public double getDimensionConverterInput() throws ParseException {
+			final Number value = NUMBER_FORMATTER.parse(this.valueInput.getText());
+			if (value instanceof Double)
+				return (double) value;
+			else if (value instanceof Long)
+				return ((Long) value).longValue();
+			else
+				throw new AssertionError();
+		}
+		
+		/**
+		 * @return selection in "From" selector in dimension-based converter
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public String getFromSelection() {
+			return this.fromSearch.getSelectedValue();
+		}
+		
+		/**
+		 * @return text in "From" box in converter panel
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public String getFromText() {
+			return this.fromEntry.getText();
+		}
+		
+		/**
+		 * @return index of selected prefix in prefix viewer
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public String getPrefixViewerSelection() {
+			return this.prefixNameList.getSelectedValue();
+		}
+		
+		/**
+		 * @return selection in "To" selector in dimension-based converter
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public String getToSelection() {
+			return this.toSearch.getSelectedValue();
+		}
+		
+		/**
+		 * @return text in "To" box in converter panel
+		 * @since 2019-01-26
+		 * @since v0.1.0
+		 */
+		public String getToText() {
+			return this.toEntry.getText();
+		}
+		
+		/**
+		 * @return index of selected unit in unit viewer
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public String getUnitViewerSelection() {
+			return this.unitNameList.getSelectedValue();
+		}
+		
+		/**
+		 * Starts up the application.
+		 * 
+		 * @since 2018-12-27
+		 * @since v0.1.0
+		 */
+		public final void init() {
+			this.frame.setVisible(true);
+		}
+		
+		/**
+		 * Initializes the view's components.
+		 * 
+		 * @since 2018-12-27
+		 * @since v0.1.0
+		 */
+		private final void initComponents() {
+			final JPanel masterPanel = new JPanel();
+			this.frame.add(masterPanel);
+			
+			masterPanel.setLayout(new BorderLayout());
+			
+			{ // pane with all of the tabs
+				masterPanel.add(this.masterPane, BorderLayout.CENTER);
+				
+				// update stuff
+				this.masterPane.addChangeListener(e -> this.update());
+				
+				{ // a panel for unit conversion using a selector
+					final JPanel convertUnitPanel = new JPanel();
+					this.masterPane.addTab("Convert Units", convertUnitPanel);
+					this.masterPane.setMnemonicAt(0, KeyEvent.VK_U);
+					
+					convertUnitPanel.setLayout(new BorderLayout());
+					
+					{ // panel for input part
+						final JPanel inputPanel = new JPanel();
+						convertUnitPanel.add(inputPanel, BorderLayout.CENTER);
+						
+						inputPanel.setLayout(new GridLayout(1, 3));
+						
+						final JComboBox<String> dimensionSelector = new JComboBox<>(
+								this.presenter.dimensionNameList()
+										.toArray(new String[0]));
+						dimensionSelector.setSelectedItem("LENGTH");
+						
+						// handle dimension filter
+						final MutablePredicate<String> dimensionFilter = new MutablePredicate<>(
+								s -> true);
+						
+						// panel for From things
+						inputPanel.add(this.fromSearch);
+						
+						this.fromSearch.addSearchFilter(dimensionFilter);
+						
+						{ // for dimension selector and arrow that represents
+							// conversion
+							final JPanel inBetweenPanel = new JPanel();
+							inputPanel.add(inBetweenPanel);
+							
+							inBetweenPanel.setLayout(new BorderLayout());
+							
+							{ // dimension selector
+								inBetweenPanel.add(dimensionSelector,
+										BorderLayout.PAGE_START);
+							}
+							
+							{ // the arrow in the middle
+								final JLabel arrowLabel = new JLabel("->");
+								inBetweenPanel.add(arrowLabel, BorderLayout.CENTER);
+							}
+						}
+						
+						// panel for To things
+						
+						inputPanel.add(this.toSearch);
+						
+						this.toSearch.addSearchFilter(dimensionFilter);
+						
+						// code for dimension filter
+						dimensionSelector.addItemListener(e -> {
+							dimensionFilter.setPredicate(string -> View.this.presenter
+									.unitMatchesDimension(string,
+											(String) dimensionSelector.getSelectedItem()));
+							this.fromSearch.reapplyFilter();
+							this.toSearch.reapplyFilter();
+						});
+						
+						// apply the item listener once because I have a default
+						// selection
+						dimensionFilter.setPredicate(string -> View.this.presenter
+								.unitMatchesDimension(string,
+										(String) dimensionSelector.getSelectedItem()));
+						this.fromSearch.reapplyFilter();
+						this.toSearch.reapplyFilter();
+					}
+					
+					{ // panel for submit and output, and also value entry
+						final JPanel outputPanel = new JPanel();
+						convertUnitPanel.add(outputPanel, BorderLayout.PAGE_END);
+						
+						outputPanel.setLayout(new GridLayout(3, 1));
+						
+						{ // unit input
+							final JPanel valueInputPanel = new JPanel();
+							outputPanel.add(valueInputPanel);
+							
+							valueInputPanel.setLayout(new BorderLayout());
+							
+							{ // prompt
+								final JLabel valuePrompt = new JLabel(
+										"Value to convert: ");
+								valueInputPanel.add(valuePrompt,
+										BorderLayout.LINE_START);
+							}
+							
+							{ // value to convert
+								valueInputPanel.add(this.valueInput,
+										BorderLayout.CENTER);
+							}
+						}
+						
+						{ // button to convert
+							final JButton convertButton = new JButton("Convert");
+							outputPanel.add(convertButton);
+							
+							convertButton.addActionListener(
+									e -> this.presenter.convertDimensionBased());
+							convertButton.setMnemonic(KeyEvent.VK_ENTER);
+						}
+						
+						{ // output of conversion
+							outputPanel.add(this.dimensionBasedOutput);
+							this.dimensionBasedOutput.setEditable(false);
+						}
+					}
+				}
+				
+				{ // panel for unit conversion using expressions
+					final JPanel convertExpressionPanel = new JPanel();
+					this.masterPane.addTab("Convert Unit Expressions",
+							convertExpressionPanel);
+					this.masterPane.setMnemonicAt(1, KeyEvent.VK_E);
+					
+					convertExpressionPanel.setLayout(new GridLayout(4, 1));
+					
+					{ // panel for units to convert from
+						final JPanel fromPanel = new JPanel();
+						convertExpressionPanel.add(fromPanel);
+						
+						fromPanel.setBorder(BorderFactory.createTitledBorder("From"));
+						fromPanel.setLayout(new GridLayout(1, 1));
+						
+						{ // entry for units
+							fromPanel.add(this.fromEntry);
+						}
+					}
+					
+					{ // panel for units to convert to
+						final JPanel toPanel = new JPanel();
+						convertExpressionPanel.add(toPanel);
+						
+						toPanel.setBorder(BorderFactory.createTitledBorder("To"));
+						toPanel.setLayout(new GridLayout(1, 1));
+						
+						{ // entry for units
+							toPanel.add(this.toEntry);
+						}
+					}
+					
+					{ // button to convert
+						final JButton convertButton = new JButton("Convert");
+						convertExpressionPanel.add(convertButton);
+						
+						convertButton.addActionListener(
+								e -> this.presenter.convertExpressions());
+						convertButton.setMnemonic(KeyEvent.VK_ENTER);
+					}
+					
+					{ // output of conversion
+						final JPanel outputPanel = new JPanel();
+						convertExpressionPanel.add(outputPanel);
+						
+						outputPanel
+								.setBorder(BorderFactory.createTitledBorder("Output"));
+						outputPanel.setLayout(new GridLayout(1, 1));
+						
+						{ // output
+							outputPanel.add(this.output);
+							this.output.setEditable(false);
+						}
+					}
+				}
+				
+				{ // panel to look up units
+					final JPanel unitLookupPanel = new JPanel();
+					this.masterPane.addTab("Unit Viewer", unitLookupPanel);
+					this.masterPane.setMnemonicAt(2, KeyEvent.VK_V);
+					
+					unitLookupPanel.setLayout(new GridLayout());
+					
+					{ // search panel
+						unitLookupPanel.add(this.unitNameList);
+						
+						this.unitNameList.getSearchList().addListSelectionListener(
+								e -> this.presenter.unitNameSelected());
+					}
+					
+					{ // the text box for unit's toString
+						unitLookupPanel.add(this.unitTextBox);
+						this.unitTextBox.setEditable(false);
+						this.unitTextBox.setLineWrap(true);
+					}
+				}
+				
+				{ // panel to look up prefixes
+					final JPanel prefixLookupPanel = new JPanel();
+					this.masterPane.addTab("Prefix Viewer", prefixLookupPanel);
+					this.masterPane.setMnemonicAt(3, KeyEvent.VK_P);
+					
+					prefixLookupPanel.setLayout(new GridLayout(1, 2));
+					
+					{ // panel for listing and seaching
+						prefixLookupPanel.add(this.prefixNameList);
+						
+						this.prefixNameList.getSearchList().addListSelectionListener(
+								e -> this.presenter.prefixSelected());
+					}
+					
+					{ // the text box for prefix's toString
+						prefixLookupPanel.add(this.prefixTextBox);
+						this.prefixTextBox.setEditable(false);
+						this.prefixTextBox.setLineWrap(true);
+					}
+				}
+				
+				{ // Info panel
+					final JPanel infoPanel = new JPanel();
+					this.masterPane.addTab("\uD83D\uDEC8", // info (i) character
+							new JScrollPane(infoPanel));
+					
+					final JTextArea infoTextArea = new JTextArea();
+					infoTextArea.setEditable(false);
+					infoTextArea.setOpaque(false);
+					infoPanel.add(infoTextArea);
+					
+					// get info text
+					final String infoText = Presenter
+							.getLinesFromResource("/about.txt").stream()
+							.map(Presenter::withoutComments)
+							.collect(Collectors.joining("\n"));
+					infoTextArea.setText(infoText);
+				}
+				
+				{ // Settings panel
+					final JPanel settingsPanel = new JPanel();
+					this.masterPane.addTab("\u2699", new JScrollPane(settingsPanel));
+					this.masterPane.setMnemonicAt(5, KeyEvent.VK_S);
+					
+					settingsPanel.setLayout(
+							new BoxLayout(settingsPanel, BoxLayout.PAGE_AXIS));
+					
+					{ // rounding settings
+						final JPanel roundingPanel = new JPanel();
+						settingsPanel.add(roundingPanel);
+						roundingPanel
+								.setBorder(new TitledBorder("Rounding Settings"));
+						roundingPanel.setLayout(new GridBagLayout());
+						
+						// rounding rule selection
+						final ButtonGroup roundingRuleButtons = new ButtonGroup();
+						
+						final JLabel roundingRuleLabel = new JLabel("Rounding Rule:");
+						roundingPanel.add(roundingRuleLabel, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedPrecision = new JRadioButton(
+								"Fixed Precision");
+						if (this.presenter.roundingType == RoundingType.SIGNIFICANT_DIGITS) {
+							fixedPrecision.setSelected(true);
+						}
+						fixedPrecision.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.SIGNIFICANT_DIGITS));
+						roundingRuleButtons.add(fixedPrecision);
+						roundingPanel.add(fixedPrecision, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedDecimals = new JRadioButton(
+								"Fixed Decimal Places");
+						if (this.presenter.roundingType == RoundingType.DECIMAL_PLACES) {
+							fixedDecimals.setSelected(true);
+						}
+						fixedDecimals.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.DECIMAL_PLACES));
+						roundingRuleButtons.add(fixedDecimals);
+						roundingPanel.add(fixedDecimals, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton relativePrecision = new JRadioButton(
+								"Scientific Precision");
+						if (this.presenter.roundingType == RoundingType.SCIENTIFIC) {
+							relativePrecision.setSelected(true);
+						}
+						relativePrecision.addActionListener(e -> this.presenter
+								.setRoundingType(RoundingType.SCIENTIFIC));
+						roundingRuleButtons.add(relativePrecision);
+						roundingPanel.add(relativePrecision, new GridBagBuilder(0, 3)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JLabel sliderLabel = new JLabel("Precision:");
+						roundingPanel.add(sliderLabel, new GridBagBuilder(0, 4)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JSlider sigDigSlider = new JSlider(0, 12);
+						roundingPanel.add(sigDigSlider, new GridBagBuilder(0, 5)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						sigDigSlider.setMajorTickSpacing(4);
+						sigDigSlider.setMinorTickSpacing(1);
+						sigDigSlider.setSnapToTicks(true);
+						sigDigSlider.setPaintTicks(true);
+						sigDigSlider.setPaintLabels(true);
+						sigDigSlider.setValue(this.presenter.precision);
+						
+						sigDigSlider.addChangeListener(e -> this.presenter
+								.setPrecision(sigDigSlider.getValue()));
+					}
+					
+					{ // prefix repetition settings
+						final JPanel prefixRepetitionPanel = new JPanel();
+						settingsPanel.add(prefixRepetitionPanel);
+						prefixRepetitionPanel.setBorder(
+								new TitledBorder("Prefix Repetition Settings"));
+						prefixRepetitionPanel.setLayout(new GridBagLayout());
+						
+						// prefix rules
+						final ButtonGroup prefixRuleButtons = new ButtonGroup();
+						
+						final JRadioButton noRepetition = new JRadioButton(
+								"No Repetition");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.NO_REPETITION) {
+							noRepetition.setSelected(true);
+						}
+						noRepetition.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.NO_REPETITION));
+						prefixRuleButtons.add(noRepetition);
+						prefixRepetitionPanel.add(noRepetition,
+								new GridBagBuilder(0, 0)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+						
+						final JRadioButton noRestriction = new JRadioButton(
+								"No Restriction");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.NO_RESTRICTION) {
+							noRestriction.setSelected(true);
+						}
+						noRestriction.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.NO_RESTRICTION));
+						prefixRuleButtons.add(noRestriction);
+						prefixRepetitionPanel.add(noRestriction,
+								new GridBagBuilder(0, 1)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+						
+						final JRadioButton customRepetition = new JRadioButton(
+								"Complex Repetition");
+						if (this.presenter.prefixRule == DefaultPrefixRepetitionRule.COMPLEX_REPETITION) {
+							customRepetition.setSelected(true);
+						}
+						customRepetition.addActionListener(
+								e -> this.presenter.setPrefixRepetitionRule(
+										DefaultPrefixRepetitionRule.COMPLEX_REPETITION));
+						prefixRuleButtons.add(customRepetition);
+						prefixRepetitionPanel.add(customRepetition,
+								new GridBagBuilder(0, 2)
+										.setAnchor(GridBagConstraints.LINE_START)
+										.build());
+					}
+					
+					{ // search settings
+						final JPanel searchingPanel = new JPanel();
+						settingsPanel.add(searchingPanel);
+						searchingPanel.setBorder(new TitledBorder("Search Settings"));
+						searchingPanel.setLayout(new GridBagLayout());
+						
+						// searching rules
+						final ButtonGroup searchRuleButtons = new ButtonGroup();
+						
+						final JRadioButton noPrefixes = new JRadioButton(
+								"Never Include Prefixed Units");
+						noPrefixes.setEnabled(false);
+						searchRuleButtons.add(noPrefixes);
+						searchingPanel.add(noPrefixes, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton fixedPrefixes = new JRadioButton(
+								"Include Some Prefixes");
+						fixedPrefixes.setEnabled(false);
+						searchRuleButtons.add(fixedPrefixes);
+						searchingPanel.add(fixedPrefixes, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton explicitPrefixes = new JRadioButton(
+								"Include Explicit Prefixes");
+						explicitPrefixes.setEnabled(false);
+						searchRuleButtons.add(explicitPrefixes);
+						searchingPanel.add(explicitPrefixes, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JRadioButton alwaysInclude = new JRadioButton(
+								"Include All Single Prefixes");
+						alwaysInclude.setEnabled(false);
+						searchRuleButtons.add(alwaysInclude);
+						searchingPanel.add(alwaysInclude, new GridBagBuilder(0, 3)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+					}
+					
+					{ // miscellaneous settings
+						final JPanel miscPanel = new JPanel();
+						settingsPanel.add(miscPanel);
+						miscPanel
+								.setBorder(new TitledBorder("Miscellaneous Settings"));
+						miscPanel.setLayout(new GridBagLayout());
+						
+						final JCheckBox oneWay = new JCheckBox(
+								"Convert One Way Only");
+						oneWay.setSelected(this.presenter.oneWay);
+						oneWay.addItemListener(
+								e -> this.presenter.setOneWay(e.getStateChange() == 1));
+						miscPanel.add(oneWay, new GridBagBuilder(0, 0)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JCheckBox showAllVariations = new JCheckBox(
+								"Show Symbols in \"Convert Units\"");
+						showAllVariations.setSelected(true);
+						showAllVariations.setEnabled(false);
+						miscPanel.add(showAllVariations, new GridBagBuilder(0, 1)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+						
+						final JButton unitFileButton = new JButton(
+								"Manage Unit Data Files");
+						unitFileButton.setEnabled(false);
+						miscPanel.add(unitFileButton, new GridBagBuilder(0, 2)
+								.setAnchor(GridBagConstraints.LINE_START).build());
+					}
+				}
+			}
+		}
+		
+		/**
+		 * Sets the text in the output of the dimension-based converter.
+		 * 
+		 * @param text text to set
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public void setDimensionConverterOutputText(final String text) {
+			this.dimensionBasedOutput.setText(text);
+		}
+		
+		/**
+		 * Sets the text in the output of the conversion panel.
+		 * 
+		 * @param text text to set
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public void setExpressionConverterOutputText(final String text) {
+			this.output.setText(text);
+		}
+		
+		/**
+		 * Sets the text of the prefix text box in the prefix viewer.
+		 * 
+		 * @param text text to set
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public void setPrefixTextBoxText(final String text) {
+			this.prefixTextBox.setText(text);
+		}
+		
+		/**
+		 * Sets the text of the unit text box in the unit viewer.
+		 * 
+		 * @param text text to set
+		 * @since 2019-01-15
+		 * @since v0.1.0
+		 */
+		public void setUnitTextBoxText(final String text) {
+			this.unitTextBox.setText(text);
+		}
+		
+		/**
+		 * Shows an error dialog.
+		 * 
+		 * @param title   title of dialog
+		 * @param message message in dialog
+		 * @since 2019-01-14
+		 * @since v0.1.0
+		 */
+		public void showErrorDialog(final String title, final String message) {
+			JOptionPane.showMessageDialog(this.frame, message, title,
+					JOptionPane.ERROR_MESSAGE);
+		}
+		
+		public void update() {
+			switch (this.getActivePane()) {
+			case UNIT_CONVERTER:
+				this.fromSearch.updateList();
+				this.toSearch.updateList();
+				break;
+			default:
+				// do nothing, for now
+				break;
+			}
+		}
+	}
+	
+	public static void main(final String[] args) {
+		new View().init();
+	}
+}
diff --git a/src/main/java/org/unitConverter/converterGUI/package-info.java b/src/main/java/org/unitConverter/converterGUI/package-info.java
new file mode 100644
index 0000000..d85ecab
--- /dev/null
+++ b/src/main/java/org/unitConverter/converterGUI/package-info.java
@@ -0,0 +1,24 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+/**
+ * The GUI interface of the Unit Converter.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-25
+ * @since v0.2.0
+ */
+package org.unitConverter.converterGUI;
\ No newline at end of file
diff --git a/src/main/java/org/unitConverter/math/ConditionalExistenceCollections.java b/src/main/java/org/unitConverter/math/ConditionalExistenceCollections.java
new file mode 100644
index 0000000..000658b
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/ConditionalExistenceCollections.java
@@ -0,0 +1,468 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.math;
+
+import java.util.AbstractCollection;
+import java.util.AbstractMap;
+import java.util.AbstractSet;
+import java.util.Collection;
+import java.util.Iterator;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.NoSuchElementException;
+import java.util.Set;
+import java.util.function.Predicate;
+
+/**
+ * Elements in these wrapper collections only exist if they pass a condition.
+ * <p>
+ * All of the collections in this class are "views" of the provided collections.
+ * They are mutable if the provided collections are mutable, they allow null if
+ * the provided collections allow null, they will reflect changes in the
+ * provided collection, etc.
+ * <p>
+ * The modification operations will always run the corresponding operations,
+ * even if the conditional existence collection doesn't change. For example, if
+ * you have a set that ignores even numbers, add(2) will still add a 2 to the
+ * backing set (but the conditional existence set will say it doesn't exist).
+ * <p>
+ * The returned collections do <i>not</i> pass the hashCode and equals
+ * operations through to the backing collections, but rely on {@code Object}'s
+ * {@code equals} and {@code hashCode} methods. This is necessary to preserve
+ * the contracts of these operations in the case that the backing collections
+ * are sets or lists.
+ * <p>
+ * Other than that, <i>the only difference between the provided collections and
+ * the returned collections are that elements don't exist if they don't pass the
+ * provided condition</i>.
+ * 
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-17
+ */
+// TODO add conditional existence Lists and Sorted/Navigable Sets/Maps
+public final class ConditionalExistenceCollections {
+	/**
+	 * Elements in this collection only exist if they meet a condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of element in collection
+	 */
+	static final class ConditionalExistenceCollection<E>
+			extends AbstractCollection<E> {
+		final Collection<E> collection;
+		final Predicate<E> existenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceCollection}.
+		 * 
+		 * @param collection
+		 * @param existenceCondition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceCollection(final Collection<E> collection,
+				final Predicate<E> existenceCondition) {
+			this.collection = collection;
+			this.existenceCondition = existenceCondition;
+		}
+		
+		@Override
+		public boolean add(final E e) {
+			return this.collection.add(e) && this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public void clear() {
+			this.collection.clear();
+		}
+		
+		@Override
+		public boolean contains(final Object o) {
+			if (!this.collection.contains(o))
+				return false;
+				
+			// this collection can only contain instances of E
+			// since the object is in the collection, we know that it must be an
+			// instance of E
+			// therefore this cast will always work
+			@SuppressWarnings("unchecked")
+			final E e = (E) o;
+			
+			return this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public Iterator<E> iterator() {
+			return conditionalExistenceIterator(this.collection.iterator(),
+					this.existenceCondition);
+		}
+		
+		@Override
+		public boolean remove(final Object o) {
+			// remove() must be first in the && statement, otherwise it may not
+			// execute
+			final boolean containedObject = this.contains(o);
+			return this.collection.remove(o) && containedObject;
+		}
+		
+		@Override
+		public int size() {
+			return (int) this.collection.stream().filter(this.existenceCondition)
+					.count();
+		}
+		
+		@Override
+		public Object[] toArray() {
+			// ensure the toArray operation is supported
+			this.collection.toArray();
+			
+			// if it works, do it for real
+			return super.toArray();
+		}
+		
+		@Override
+		public <T> T[] toArray(T[] a) {
+			// ensure the toArray operation is supported
+			this.collection.toArray();
+			
+			// if it works, do it for real
+			return super.toArray(a);
+		}
+	}
+	
+	/**
+	 * Elements in this wrapper iterator only exist if they pass a condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of elements in iterator
+	 */
+	static final class ConditionalExistenceIterator<E> implements Iterator<E> {
+		final Iterator<E> iterator;
+		final Predicate<E> existenceCondition;
+		E nextElement;
+		boolean hasNext;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceIterator}.
+		 * 
+		 * @param iterator
+		 * @param condition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceIterator(final Iterator<E> iterator,
+				final Predicate<E> condition) {
+			this.iterator = iterator;
+			this.existenceCondition = condition;
+			this.getAndSetNextElement();
+		}
+		
+		/**
+		 * Gets the next element, and sets nextElement and hasNext accordingly.
+		 * 
+		 * @since 2019-10-17
+		 */
+		private void getAndSetNextElement() {
+			do {
+				if (!this.iterator.hasNext()) {
+					this.nextElement = null;
+					this.hasNext = false;
+					return;
+				}
+				this.nextElement = this.iterator.next();
+			} while (!this.existenceCondition.test(this.nextElement));
+			this.hasNext = true;
+		}
+		
+		@Override
+		public boolean hasNext() {
+			return this.hasNext;
+		}
+		
+		@Override
+		public E next() {
+			if (this.hasNext()) {
+				final E next = this.nextElement;
+				this.getAndSetNextElement();
+				return next;
+			} else
+				throw new NoSuchElementException();
+		}
+		
+		@Override
+		public void remove() {
+			this.iterator.remove();
+		}
+	}
+	
+	/**
+	 * Mappings in this map only exist if the entry passes some condition.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <K> key type
+	 * @param <V> value type
+	 */
+	static final class ConditionalExistenceMap<K, V> extends AbstractMap<K, V> {
+		Map<K, V> map;
+		Predicate<Entry<K, V>> entryExistenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalExistenceMap}.
+		 * 
+		 * @param map
+		 * @param entryExistenceCondition
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceMap(final Map<K, V> map,
+				final Predicate<Entry<K, V>> entryExistenceCondition) {
+			this.map = map;
+			this.entryExistenceCondition = entryExistenceCondition;
+		}
+		
+		@Override
+		public boolean containsKey(final Object key) {
+			if (!this.map.containsKey(key))
+				return false;
+				
+			// only instances of K have mappings in the backing map
+			// since we know that key is a valid key, it must be an instance of K
+			@SuppressWarnings("unchecked")
+			final K keyAsK = (K) key;
+			
+			// get and test entry
+			final V value = this.map.get(key);
+			final Entry<K, V> entry = new SimpleEntry<>(keyAsK, value);
+			return this.entryExistenceCondition.test(entry);
+		}
+		
+		@Override
+		public Set<Entry<K, V>> entrySet() {
+			return conditionalExistenceSet(this.map.entrySet(),
+					this.entryExistenceCondition);
+		}
+		
+		@Override
+		public V get(final Object key) {
+			return this.containsKey(key) ? this.map.get(key) : null;
+		}
+		
+		private final Entry<K, V> getEntry(K key) {
+			return new Entry<>() {
+				@Override
+				public K getKey() {
+					return key;
+				}
+				
+				@Override
+				public V getValue() {
+					return ConditionalExistenceMap.this.map.get(key);
+				}
+				
+				@Override
+				public V setValue(V value) {
+					return ConditionalExistenceMap.this.map.put(key, value);
+				}
+			};
+		}
+		
+		@Override
+		public Set<K> keySet() {
+			return conditionalExistenceSet(super.keySet(),
+					k -> this.entryExistenceCondition.test(this.getEntry(k)));
+		}
+		
+		@Override
+		public V put(final K key, final V value) {
+			final V oldValue = this.map.put(key, value);
+			
+			// get and test entry
+			final Entry<K, V> entry = new SimpleEntry<>(key, oldValue);
+			return this.entryExistenceCondition.test(entry) ? oldValue : null;
+		}
+		
+		@Override
+		public V remove(final Object key) {
+			final V oldValue = this.map.remove(key);
+			return this.containsKey(key) ? oldValue : null;
+		}
+		
+		@Override
+		public Collection<V> values() {
+			// maybe change this to use ConditionalExistenceCollection
+			return super.values();
+		}
+	}
+	
+	/**
+	 * Elements in this set only exist if a certain condition is true.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-17
+	 * @param <E> type of element in set
+	 */
+	static final class ConditionalExistenceSet<E> extends AbstractSet<E> {
+		private final Set<E> set;
+		private final Predicate<E> existenceCondition;
+		
+		/**
+		 * Creates the {@code ConditionalNonexistenceSet}.
+		 * 
+		 * @param set                set to use
+		 * @param existenceCondition condition where element exists
+		 * @since 2019-10-17
+		 */
+		private ConditionalExistenceSet(final Set<E> set,
+				final Predicate<E> existenceCondition) {
+			this.set = set;
+			this.existenceCondition = existenceCondition;
+		}
+		
+		/**
+		 * {@inheritDoc}
+		 * <p>
+		 * Note that this method returns {@code false} if {@code e} does not pass
+		 * the existence condition.
+		 */
+		@Override
+		public boolean add(final E e) {
+			return this.set.add(e) && this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public void clear() {
+			this.set.clear();
+		}
+		
+		@Override
+		public boolean contains(final Object o) {
+			if (!this.set.contains(o))
+				return false;
+				
+			// this set can only contain instances of E
+			// since the object is in the set, we know that it must be an instance
+			// of E
+			// therefore this cast will always work
+			@SuppressWarnings("unchecked")
+			final E e = (E) o;
+			
+			return this.existenceCondition.test(e);
+		}
+		
+		@Override
+		public Iterator<E> iterator() {
+			return conditionalExistenceIterator(this.set.iterator(),
+					this.existenceCondition);
+		}
+		
+		@Override
+		public boolean remove(final Object o) {
+			// remove() must be first in the && statement, otherwise it may not
+			// execute
+			final boolean containedObject = this.contains(o);
+			return this.set.remove(o) && containedObject;
+		}
+		
+		@Override
+		public int size() {
+			return (int) this.set.stream().filter(this.existenceCondition).count();
+		}
+		
+		@Override
+		public Object[] toArray() {
+			// ensure the toArray operation is supported
+			this.set.toArray();
+			
+			// if it works, do it for real
+			return super.toArray();
+		}
+		
+		@Override
+		public <T> T[] toArray(T[] a) {
+			// ensure the toArray operation is supported
+			this.set.toArray();
+			
+			// if it works, do it for real
+			return super.toArray(a);
+		}
+	}
+	
+	/**
+	 * Elements in the returned wrapper collection are ignored if they don't pass
+	 * a condition.
+	 * 
+	 * @param <E>                type of elements in collection
+	 * @param collection         collection to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper collection
+	 * @since 2019-10-17
+	 */
+	public static final <E> Collection<E> conditionalExistenceCollection(
+			final Collection<E> collection,
+			final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceCollection<>(collection,
+				existenceCondition);
+	}
+	
+	/**
+	 * Elements in the returned wrapper iterator are ignored if they don't pass a
+	 * condition.
+	 * 
+	 * @param <E>                type of elements in iterator
+	 * @param iterator           iterator to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper iterator
+	 * @since 2019-10-17
+	 */
+	public static final <E> Iterator<E> conditionalExistenceIterator(
+			final Iterator<E> iterator, final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceIterator<>(iterator, existenceCondition);
+	}
+	
+	/**
+	 * Mappings in the returned wrapper map are ignored if the corresponding
+	 * entry doesn't pass a condition
+	 * 
+	 * @param <K>                     type of key in map
+	 * @param <V>                     type of value in map
+	 * @param map                     map to wrap
+	 * @param entryExistenceCondition mappings only exist if this returns true
+	 * @return wrapper map
+	 * @since 2019-10-17
+	 */
+	public static final <K, V> Map<K, V> conditionalExistenceMap(
+			final Map<K, V> map,
+			final Predicate<Entry<K, V>> entryExistenceCondition) {
+		return new ConditionalExistenceMap<>(map, entryExistenceCondition);
+	}
+	
+	/**
+	 * Elements in the returned wrapper set are ignored if they don't pass a
+	 * condition.
+	 * 
+	 * @param <E>                type of elements in set
+	 * @param set                set to wrap
+	 * @param existenceCondition elements only exist if this returns true
+	 * @return wrapper set
+	 * @since 2019-10-17
+	 */
+	public static final <E> Set<E> conditionalExistenceSet(final Set<E> set,
+			final Predicate<E> existenceCondition) {
+		return new ConditionalExistenceSet<>(set, existenceCondition);
+	}
+}
diff --git a/src/main/java/org/unitConverter/math/DecimalComparison.java b/src/main/java/org/unitConverter/math/DecimalComparison.java
new file mode 100644
index 0000000..0f5b91e
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/DecimalComparison.java
@@ -0,0 +1,256 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.math;
+
+import java.math.BigDecimal;
+
+/**
+ * A class that contains methods to compare float and double values.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-03-18
+ * @since v0.2.0
+ */
+public final class DecimalComparison {
+	/**
+	 * The value used for double comparison. If two double values are within this
+	 * value multiplied by the larger value, they are considered equal.
+	 * 
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final double DOUBLE_EPSILON = 1.0e-15;
+	
+	/**
+	 * The value used for float comparison. If two float values are within this
+	 * value multiplied by the larger value, they are considered equal.
+	 * 
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final float FLOAT_EPSILON = 1.0e-6f;
+	
+	/**
+	 * Tests for equality of double values using {@link #DOUBLE_EPSILON}.
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon using
+	 * {@link #equals(double, double, double)} (this does not make a violation of
+	 * transitivity impossible, it just significantly reduces the chances of it
+	 * happening)
+	 * <li>Use {@link BigDecimal} instead of {@code double} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a first value to test
+	 * @param b second value to test
+	 * @return whether they are equal
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 * @see #hashCode(double)
+	 */
+	public static final boolean equals(final double a, final double b) {
+		return DecimalComparison.equals(a, b, DOUBLE_EPSILON);
+	}
+	
+	/**
+	 * Tests for double equality using a custom epsilon value.
+	 * 
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon (this does not make a violation of
+	 * transitivity impossible, it just significantly reduces the chances of it
+	 * happening)
+	 * <li>Use {@link BigDecimal} instead of {@code double} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a       first value to test
+	 * @param b       second value to test
+	 * @param epsilon allowed difference
+	 * @return whether they are equal
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final boolean equals(final double a, final double b,
+			final double epsilon) {
+		return Math.abs(a - b) <= epsilon * Math.max(Math.abs(a), Math.abs(b));
+	}
+	
+	/**
+	 * Tests for equality of float values using {@link #FLOAT_EPSILON}.
+	 * 
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon using {@link #equals(float, float, float)}
+	 * (this does not make a violation of transitivity impossible, it just
+	 * significantly reduces the chances of it happening)
+	 * <li>Use {@link BigDecimal} instead of {@code float} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a first value to test
+	 * @param b second value to test
+	 * @return whether they are equal
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final boolean equals(final float a, final float b) {
+		return DecimalComparison.equals(a, b, FLOAT_EPSILON);
+	}
+	
+	/**
+	 * Tests for float equality using a custom epsilon value.
+	 * 
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon (this does not make a violation of
+	 * transitivity impossible, it just significantly reduces the chances of it
+	 * happening)
+	 * <li>Use {@link BigDecimal} instead of {@code float} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a       first value to test
+	 * @param b       second value to test
+	 * @param epsilon allowed difference
+	 * @return whether they are equal
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final boolean equals(final float a, final float b,
+			final float epsilon) {
+		return Math.abs(a - b) <= epsilon * Math.max(Math.abs(a), Math.abs(b));
+	}
+	
+	/**
+	 * Tests for equality of {@code UncertainDouble} values using
+	 * {@link #DOUBLE_EPSILON}.
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon using
+	 * {@link #equals(UncertainDouble, UncertainDouble, double)} (this does not
+	 * make a violation of transitivity impossible, it just significantly reduces
+	 * the chances of it happening)
+	 * <li>Use {@link BigDecimal} instead of {@code double} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a first value to test
+	 * @param b second value to test
+	 * @return whether they are equal
+	 * @since 2020-09-07
+	 * @see #hashCode(double)
+	 */
+	public static final boolean equals(final UncertainDouble a,
+			final UncertainDouble b) {
+		return DecimalComparison.equals(a.value(), b.value())
+				&& DecimalComparison.equals(a.uncertainty(), b.uncertainty());
+	}
+	
+	/**
+	 * Tests for {@code UncertainDouble} equality using a custom epsilon value.
+	 * 
+	 * <p>
+	 * <strong>WARNING: </strong>this method is not technically transitive. If a
+	 * and b are off by slightly less than {@code epsilon * max(abs(a), abs(b))},
+	 * and b and c are off by slightly less than
+	 * {@code epsilon * max(abs(b), abs(c))}, then equals(a, b) and equals(b, c)
+	 * will both return true, but equals(a, c) will return false. However, this
+	 * situation is very unlikely to ever happen in a real programming situation.
+	 * <p>
+	 * If this does become a concern, some ways to solve this problem:
+	 * <ol>
+	 * <li>Raise the value of epsilon (this does not make a violation of
+	 * transitivity impossible, it just significantly reduces the chances of it
+	 * happening)
+	 * <li>Use {@link BigDecimal} instead of {@code double} (this will make a
+	 * violation of transitivity 100% impossible)
+	 * </ol>
+	 * 
+	 * @param a       first value to test
+	 * @param b       second value to test
+	 * @param epsilon allowed difference
+	 * @return whether they are equal
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 */
+	public static final boolean equals(final UncertainDouble a,
+			final UncertainDouble b, final double epsilon) {
+		return DecimalComparison.equals(a.value(), b.value(), epsilon)
+				&& DecimalComparison.equals(a.uncertainty(), b.uncertainty(),
+						epsilon);
+	}
+	
+	/**
+	 * Takes the hash code of doubles. Values that are equal according to
+	 * {@link #equals(double, double)} will have the same hash code.
+	 * 
+	 * @param d double to hash
+	 * @return hash code of double
+	 * @since 2019-10-16
+	 */
+	public static final int hash(final double d) {
+		return Float.hashCode((float) d);
+	}
+	
+	// You may NOT get any DecimalComparison instances
+	private DecimalComparison() {
+		throw new AssertionError();
+	}
+	
+}
diff --git a/src/main/java/org/unitConverter/math/ExpressionParser.java b/src/main/java/org/unitConverter/math/ExpressionParser.java
new file mode 100644
index 0000000..deee51d
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/ExpressionParser.java
@@ -0,0 +1,735 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.math;
+
+import java.util.ArrayDeque;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Deque;
+import java.util.HashMap;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.function.BinaryOperator;
+import java.util.function.Function;
+import java.util.function.UnaryOperator;
+
+/**
+ * An object that can parse expressions with unary or binary operators.
+ * 
+ * @author Adrien Hopkins
+ * @param <T> type of object that exists in parsed expressions
+ * @since 2019-03-14
+ * @since v0.2.0
+ */
+public final class ExpressionParser<T> {
+	/**
+	 * A builder that can create {@code ExpressionParser<T>} instances.
+	 * 
+	 * @author Adrien Hopkins
+	 * @param <T> type of object that exists in parsed expressions
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	public static final class Builder<T> {
+		/**
+		 * A function that obtains a parseable object from a string. For example,
+		 * an integer {@code ExpressionParser} would use
+		 * {@code Integer::parseInt}.
+		 * 
+		 * @since 2019-03-14
+		 * @since v0.2.0
+		 */
+		private final Function<String, ? extends T> objectObtainer;
+		
+		/**
+		 * The function of the space as an operator (like 3 x y)
+		 * 
+		 * @since 2019-03-22
+		 * @since v0.2.0
+		 */
+		private String spaceFunction = null;
+		
+		/**
+		 * A map mapping operator strings to operator functions, for unary
+		 * operators.
+		 * 
+		 * @since 2019-03-14
+		 * @since v0.2.0
+		 */
+		private final Map<String, PriorityUnaryOperator<T>> unaryOperators;
+		
+		/**
+		 * A map mapping operator strings to operator functions, for binary
+		 * operators.
+		 * 
+		 * @since 2019-03-14
+		 * @since v0.2.0
+		 */
+		private final Map<String, PriorityBinaryOperator<T>> binaryOperators;
+		
+		/**
+		 * Creates the {@code Builder}.
+		 * 
+		 * @param objectObtainer a function that can turn strings into objects of
+		 *                       the type handled by the parser.
+		 * @throws NullPointerException if {@code objectObtainer} is null
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public Builder(final Function<String, ? extends T> objectObtainer) {
+			this.objectObtainer = Objects.requireNonNull(objectObtainer,
+					"objectObtainer must not be null.");
+			this.unaryOperators = new HashMap<>();
+			this.binaryOperators = new HashMap<>();
+		}
+		
+		/**
+		 * Adds a binary operator to the builder.
+		 * 
+		 * @param text     text used to reference the operator, like '+'
+		 * @param operator operator to add
+		 * @param priority operator's priority, which determines which operators
+		 *                 are applied first
+		 * @return this builder
+		 * @throws NullPointerException if {@code text} or {@code operator} is
+		 *                              null
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public Builder<T> addBinaryOperator(final String text,
+				final BinaryOperator<T> operator, final int priority) {
+			Objects.requireNonNull(text, "text must not be null.");
+			Objects.requireNonNull(operator, "operator must not be null.");
+			
+			// Unfortunately, I cannot use a lambda because the
+			// PriorityBinaryOperator requires arguments.
+			final PriorityBinaryOperator<T> priorityOperator = new PriorityBinaryOperator<>(
+					priority) {
+				@Override
+				public T apply(final T t, final T u) {
+					return operator.apply(t, u);
+				}
+				
+			};
+			this.binaryOperators.put(text, priorityOperator);
+			return this;
+		}
+		
+		/**
+		 * Adds a function for spaces. You must use the text of an existing binary
+		 * operator.
+		 * 
+		 * @param operator text of operator to use
+		 * @return this builder
+		 * @since 2019-03-22
+		 * @since v0.2.0
+		 */
+		public Builder<T> addSpaceFunction(final String operator) {
+			Objects.requireNonNull(operator, "operator must not be null.");
+			
+			if (!this.binaryOperators.containsKey(operator))
+				throw new IllegalArgumentException(String
+						.format("Could not find binary operator '%s'", operator));
+			
+			this.spaceFunction = operator;
+			return this;
+		}
+		
+		/**
+		 * Adds a unary operator to the builder.
+		 * 
+		 * @param text     text used to reference the operator, like '-'
+		 * @param operator operator to add
+		 * @param priority operator's priority, which determines which operators
+		 *                 are applied first
+		 * @return this builder
+		 * @throws NullPointerException if {@code text} or {@code operator} is
+		 *                              null
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public Builder<T> addUnaryOperator(final String text,
+				final UnaryOperator<T> operator, final int priority) {
+			Objects.requireNonNull(text, "text must not be null.");
+			Objects.requireNonNull(operator, "operator must not be null.");
+			
+			// Unfortunately, I cannot use a lambda because the
+			// PriorityUnaryOperator requires arguments.
+			final PriorityUnaryOperator<T> priorityOperator = new PriorityUnaryOperator<>(
+					priority) {
+				@Override
+				public T apply(final T t) {
+					return operator.apply(t);
+				}
+			};
+			this.unaryOperators.put(text, priorityOperator);
+			return this;
+		}
+		
+		/**
+		 * @return an {@code ExpressionParser<T>} instance with the properties
+		 *         given to this builder
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public ExpressionParser<T> build() {
+			return new ExpressionParser<>(this.objectObtainer, this.unaryOperators,
+					this.binaryOperators, this.spaceFunction);
+		}
+	}
+	
+	/**
+	 * A binary operator with a priority field that determines which operators
+	 * apply first.
+	 * 
+	 * @author Adrien Hopkins
+	 * @param <T> type of operand and result
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	private static abstract class PriorityBinaryOperator<T>
+			implements BinaryOperator<T>, Comparable<PriorityBinaryOperator<T>> {
+		/**
+		 * The operator's priority. Higher-priority operators are applied before
+		 * lower-priority operators
+		 * 
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		private final int priority;
+		
+		/**
+		 * Creates the {@code PriorityBinaryOperator}.
+		 * 
+		 * @param priority operator's priority
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public PriorityBinaryOperator(final int priority) {
+			this.priority = priority;
+		}
+		
+		/**
+		 * Compares this object to another by priority.
+		 * 
+		 * <p>
+		 * {@inheritDoc}
+		 * </p>
+		 * 
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		@Override
+		public int compareTo(final PriorityBinaryOperator<T> o) {
+			if (this.priority < o.priority)
+				return -1;
+			else if (this.priority > o.priority)
+				return 1;
+			else
+				return 0;
+		}
+		
+		/**
+		 * @return priority
+		 * @since 2019-03-22
+		 * @since v0.2.0
+		 */
+		public final int getPriority() {
+			return this.priority;
+		}
+	}
+	
+	/**
+	 * A unary operator with a priority field that determines which operators
+	 * apply first.
+	 * 
+	 * @author Adrien Hopkins
+	 * @param <T> type of operand and result
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	private static abstract class PriorityUnaryOperator<T>
+			implements UnaryOperator<T>, Comparable<PriorityUnaryOperator<T>> {
+		/**
+		 * The operator's priority. Higher-priority operators are applied before
+		 * lower-priority operators
+		 * 
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		private final int priority;
+		
+		/**
+		 * Creates the {@code PriorityUnaryOperator}.
+		 * 
+		 * @param priority operator's priority
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		public PriorityUnaryOperator(final int priority) {
+			this.priority = priority;
+		}
+		
+		/**
+		 * Compares this object to another by priority.
+		 * 
+		 * <p>
+		 * {@inheritDoc}
+		 * </p>
+		 * 
+		 * @since 2019-03-17
+		 * @since v0.2.0
+		 */
+		@Override
+		public int compareTo(final PriorityUnaryOperator<T> o) {
+			if (this.priority < o.priority)
+				return -1;
+			else if (this.priority > o.priority)
+				return 1;
+			else
+				return 0;
+		}
+		
+		/**
+		 * @return priority
+		 * @since 2019-03-22
+		 * @since v0.2.0
+		 */
+		public final int getPriority() {
+			return this.priority;
+		}
+	}
+	
+	/**
+	 * The types of tokens that are available.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private static enum TokenType {
+		OBJECT, UNARY_OPERATOR, BINARY_OPERATOR;
+	}
+	
+	/**
+	 * The opening bracket.
+	 * 
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	public static final char OPENING_BRACKET = '(';
+	
+	/**
+	 * The closing bracket.
+	 * 
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	public static final char CLOSING_BRACKET = ')';
+	
+	/**
+	 * Finds the other bracket in a pair of brackets, given the position of one.
+	 * 
+	 * @param string          string that contains brackets
+	 * @param bracketPosition position of first bracket
+	 * @return position of matching bracket
+	 * @throws NullPointerException if string is null
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private static int findBracketPair(final String string,
+			final int bracketPosition) {
+		Objects.requireNonNull(string, "string must not be null.");
+		
+		final char openingBracket = string.charAt(bracketPosition);
+		
+		// figure out what closing bracket to look for
+		final char closingBracket;
+		switch (openingBracket) {
+		case '(':
+			closingBracket = ')';
+			break;
+		case '[':
+			closingBracket = ']';
+			break;
+		case '{':
+			closingBracket = '}';
+			break;
+		default:
+			throw new IllegalArgumentException(
+					String.format("Invalid bracket '%s'", openingBracket));
+		}
+		
+		// level of brackets. every opening bracket increments this; every closing
+		// bracket decrements it
+		int bracketLevel = 0;
+		
+		// iterate over the string to find the closing bracket
+		for (int currentPosition = bracketPosition; currentPosition < string
+				.length(); currentPosition++) {
+			final char currentCharacter = string.charAt(currentPosition);
+			
+			if (currentCharacter == openingBracket) {
+				bracketLevel++;
+			} else if (currentCharacter == closingBracket) {
+				bracketLevel--;
+				if (bracketLevel == 0)
+					return currentPosition;
+			}
+		}
+		
+		throw new IllegalArgumentException("No matching bracket found.");
+	}
+	
+	/**
+	 * A function that obtains a parseable object from a string. For example, an
+	 * integer {@code ExpressionParser} would use {@code Integer::parseInt}.
+	 * 
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private final Function<String, ? extends T> objectObtainer;
+	
+	/**
+	 * A map mapping operator strings to operator functions, for unary operators.
+	 * 
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private final Map<String, PriorityUnaryOperator<T>> unaryOperators;
+	
+	/**
+	 * A map mapping operator strings to operator functions, for binary
+	 * operators.
+	 * 
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private final Map<String, PriorityBinaryOperator<T>> binaryOperators;
+	
+	/**
+	 * The operator for space, or null if spaces have no function.
+	 * 
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private final String spaceOperator;
+	
+	/**
+	 * Creates the {@code ExpressionParser}.
+	 * 
+	 * @param objectObtainer  function to get objects from strings
+	 * @param unaryOperators  unary operators available to the parser
+	 * @param binaryOperators binary operators available to the parser
+	 * @param spaceOperator   operator used by spaces
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private ExpressionParser(final Function<String, ? extends T> objectObtainer,
+			final Map<String, PriorityUnaryOperator<T>> unaryOperators,
+			final Map<String, PriorityBinaryOperator<T>> binaryOperators,
+			final String spaceOperator) {
+		this.objectObtainer = objectObtainer;
+		this.unaryOperators = unaryOperators;
+		this.binaryOperators = binaryOperators;
+		this.spaceOperator = spaceOperator;
+	}
+	
+	/**
+	 * Converts a given mathematical expression to reverse Polish notation
+	 * (operators after operands).
+	 * <p>
+	 * For example,<br>
+	 * {@code 2 * (3 + 4)}<br>
+	 * becomes<br>
+	 * {@code 2 3 4 + *}.
+	 * 
+	 * @param expression expression
+	 * @return expression in RPN
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	private String convertExpressionToReversePolish(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		final List<String> components = new ArrayList<>();
+		
+		// the part of the expression remaining to parse
+		String partialExpression = expression;
+		
+		// find and deal with brackets
+		while (partialExpression.indexOf(OPENING_BRACKET) != -1) {
+			final int openingBracketPosition = partialExpression
+					.indexOf(OPENING_BRACKET);
+			final int closingBracketPosition = findBracketPair(partialExpression,
+					openingBracketPosition);
+			
+			// check for function
+			if (openingBracketPosition > 0
+					&& partialExpression.charAt(openingBracketPosition - 1) != ' ') {
+				// function like sin(2) or tempF(32)
+				// find the position of the last space
+				int spacePosition = openingBracketPosition;
+				while (spacePosition >= 0
+						&& partialExpression.charAt(spacePosition) != ' ') {
+					spacePosition--;
+				}
+				// then split the function into pre-function and function, using the
+				// space position
+				components.addAll(Arrays.asList(partialExpression
+						.substring(0, spacePosition + 1).split(" ")));
+				components.add(partialExpression.substring(spacePosition + 1,
+						closingBracketPosition + 1));
+				partialExpression = partialExpression
+						.substring(closingBracketPosition + 1);
+			} else {
+				// normal brackets like (1 + 2) * (3 / 5)
+				components.addAll(Arrays.asList(partialExpression
+						.substring(0, openingBracketPosition).split(" ")));
+				components.add(this.convertExpressionToReversePolish(
+						partialExpression.substring(openingBracketPosition + 1,
+								closingBracketPosition)));
+				partialExpression = partialExpression
+						.substring(closingBracketPosition + 1);
+			}
+		}
+		
+		// add everything else
+		components.addAll(Arrays.asList(partialExpression.split(" ")));
+		
+		// remove empty entries
+		while (components.contains("")) {
+			components.remove("");
+		}
+		
+		// deal with space multiplication (x y)
+		if (this.spaceOperator != null) {
+			for (int i = 0; i < components.size() - 1; i++) {
+				if (this.getTokenType(components.get(i)) == TokenType.OBJECT && this
+						.getTokenType(components.get(i + 1)) == TokenType.OBJECT) {
+					components.add(++i, this.spaceOperator);
+				}
+			}
+		}
+		
+		// turn the expression into reverse Polish
+		while (true) {
+			final int highestPriorityOperatorPosition = this
+					.findHighestPriorityOperatorPosition(components);
+			if (highestPriorityOperatorPosition == -1) {
+				break;
+			}
+			
+			// swap components based on what kind of operator there is
+			// 1 + 2 becomes 2 1 +
+			// - 1 becomes 1 -
+			switch (this
+					.getTokenType(components.get(highestPriorityOperatorPosition))) {
+			case UNARY_OPERATOR:
+				final String unaryOperator = components
+						.remove(highestPriorityOperatorPosition);
+				final String operand = components
+						.remove(highestPriorityOperatorPosition);
+				components.add(highestPriorityOperatorPosition,
+						operand + " " + unaryOperator);
+				break;
+			case BINARY_OPERATOR:
+				final String binaryOperator = components
+						.remove(highestPriorityOperatorPosition);
+				final String operand1 = components
+						.remove(highestPriorityOperatorPosition - 1);
+				final String operand2 = components
+						.remove(highestPriorityOperatorPosition - 1);
+				components.add(highestPriorityOperatorPosition - 1,
+						operand2 + " " + operand1 + " " + binaryOperator);
+				break;
+			default:
+				throw new AssertionError("Expected operator, found non-operator.");
+			}
+		}
+		
+		// join all of the components together, then ensure there is only one
+		// space in a row
+		String expressionRPN = String.join(" ", components).replaceAll(" +", " ");
+		
+		while (expressionRPN.charAt(0) == ' ') {
+			expressionRPN = expressionRPN.substring(1);
+		}
+		while (expressionRPN.charAt(expressionRPN.length() - 1) == ' ') {
+			expressionRPN = expressionRPN.substring(0, expressionRPN.length() - 1);
+		}
+		return expressionRPN;
+	}
+	
+	/**
+	 * Finds the position of the highest-priority operator in a list
+	 * 
+	 * @param components components to test
+	 * @param blacklist  positions of operators that should be ignored
+	 * @return position of highest priority, or -1 if the list contains no
+	 *         operators
+	 * @throws NullPointerException if components is null
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private int findHighestPriorityOperatorPosition(
+			final List<String> components) {
+		Objects.requireNonNull(components, "components must not be null.");
+		// find highest priority
+		int maxPriority = Integer.MIN_VALUE;
+		int maxPriorityPosition = -1;
+		
+		// go over components one by one
+		// if it is an operator, test its priority to see if it's max
+		// if it is, update maxPriority and maxPriorityPosition
+		for (int i = 0; i < components.size(); i++) {
+			
+			switch (this.getTokenType(components.get(i))) {
+			case UNARY_OPERATOR:
+				final PriorityUnaryOperator<T> unaryOperator = this.unaryOperators
+						.get(components.get(i));
+				final int unaryPriority = unaryOperator.getPriority();
+				
+				if (unaryPriority > maxPriority) {
+					maxPriority = unaryPriority;
+					maxPriorityPosition = i;
+				}
+				break;
+			case BINARY_OPERATOR:
+				final PriorityBinaryOperator<T> binaryOperator = this.binaryOperators
+						.get(components.get(i));
+				final int binaryPriority = binaryOperator.getPriority();
+				
+				if (binaryPriority > maxPriority) {
+					maxPriority = binaryPriority;
+					maxPriorityPosition = i;
+				}
+				break;
+			default:
+				break;
+			}
+		}
+		
+		// max priority position found
+		return maxPriorityPosition;
+	}
+	
+	/**
+	 * Determines whether an inputted string is an object or an operator
+	 * 
+	 * @param token string to input
+	 * @return type of token it is
+	 * @throws NullPointerException if {@code expression} is null
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private TokenType getTokenType(final String token) {
+		Objects.requireNonNull(token, "token must not be null.");
+		
+		if (this.unaryOperators.containsKey(token))
+			return TokenType.UNARY_OPERATOR;
+		else if (this.binaryOperators.containsKey(token))
+			return TokenType.BINARY_OPERATOR;
+		else
+			return TokenType.OBJECT;
+	}
+	
+	/**
+	 * Parses an expression.
+	 * 
+	 * @param expression expression to parse
+	 * @return result
+	 * @throws NullPointerException if {@code expression} is null
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public T parseExpression(final String expression) {
+		return this.parseReversePolishExpression(
+				this.convertExpressionToReversePolish(expression));
+	}
+	
+	/**
+	 * Parses an expression expressed in reverse Polish notation.
+	 * 
+	 * @param expression expression to parse
+	 * @return result
+	 * @throws NullPointerException if {@code expression} is null
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private T parseReversePolishExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		final Deque<T> stack = new ArrayDeque<>();
+		
+		// iterate over every item in the expression, then
+		for (final String item : expression.split(" ")) {
+			// choose a path based on what kind of thing was just read
+			switch (this.getTokenType(item)) {
+			
+			case BINARY_OPERATOR:
+				if (stack.size() < 2)
+					throw new IllegalStateException(String.format(
+							"Attempted to call binary operator %s with only %d arguments.",
+							item, stack.size()));
+				
+				// get two arguments and operator, then apply!
+				final T o1 = stack.pop();
+				final T o2 = stack.pop();
+				final BinaryOperator<T> binaryOperator = this.binaryOperators
+						.get(item);
+				
+				stack.push(binaryOperator.apply(o1, o2));
+				break;
+			
+			case OBJECT:
+				// just add it to the stack
+				stack.push(this.objectObtainer.apply(item));
+				break;
+			
+			case UNARY_OPERATOR:
+				if (stack.size() < 1)
+					throw new IllegalStateException(String.format(
+							"Attempted to call unary operator %s with only %d arguments.",
+							item, stack.size()));
+				
+				// get one argument and operator, then apply!
+				final T o = stack.pop();
+				final UnaryOperator<T> unaryOperator = this.unaryOperators
+						.get(item);
+				
+				stack.push(unaryOperator.apply(o));
+				break;
+			default:
+				throw new AssertionError(
+						String.format("Internal error: Invalid token type %s.",
+								this.getTokenType(item)));
+			
+			}
+		}
+		
+		// return answer, or throw an exception if I can't
+		if (stack.size() > 1)
+			throw new IllegalStateException(
+					"Computation ended up with more than one answer.");
+		else if (stack.size() == 0)
+			throw new IllegalStateException(
+					"Computation ended up without an answer.");
+		return stack.pop();
+	}
+}
diff --git a/src/main/java/org/unitConverter/math/ObjectProduct.java b/src/main/java/org/unitConverter/math/ObjectProduct.java
new file mode 100644
index 0000000..5217d93
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/ObjectProduct.java
@@ -0,0 +1,284 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.math;
+
+import java.util.ArrayList;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.List;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.function.Function;
+
+/**
+ * An immutable product of multiple objects of a type, such as base units. The objects can be multiplied and
+ * exponentiated.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class ObjectProduct<T> {
+	/**
+	 * Returns an empty ObjectProduct of a certain type
+	 * 
+	 * @param <T>
+	 *            type of objects that can be multiplied
+	 * @return empty product
+	 * @since 2019-10-16
+	 */
+	public static final <T> ObjectProduct<T> empty() {
+		return new ObjectProduct<>(new HashMap<>());
+	}
+
+	/**
+	 * Gets an {@code ObjectProduct} from an object-to-integer mapping
+	 * 
+	 * @param <T>
+	 *            type of object in product
+	 * @param map
+	 *            map mapping objects to exponents
+	 * @return object product
+	 * @since 2019-10-16
+	 */
+	public static final <T> ObjectProduct<T> fromExponentMapping(final Map<T, Integer> map) {
+		return new ObjectProduct<>(new HashMap<>(map));
+	}
+
+	/**
+	 * Gets an ObjectProduct that has one of the inputted argument, and nothing else.
+	 * 
+	 * @param object
+	 *            object that will be in the product
+	 * @return product
+	 * @since 2019-10-16
+	 * @throws NullPointerException
+	 *             if object is null
+	 */
+	public static final <T> ObjectProduct<T> oneOf(final T object) {
+		Objects.requireNonNull(object, "object must not be null.");
+		final Map<T, Integer> map = new HashMap<>();
+		map.put(object, 1);
+		return new ObjectProduct<>(map);
+	}
+
+	/**
+	 * The objects that make up the product, mapped to their exponents. This map treats zero as null, and is immutable.
+	 * 
+	 * @since 2019-10-16
+	 */
+	final Map<T, Integer> exponents;
+
+	/**
+	 * Creates the {@code ObjectProduct}.
+	 * 
+	 * @param exponents
+	 *            objects that make up this product
+	 * @since 2019-10-16
+	 */
+	private ObjectProduct(final Map<T, Integer> exponents) {
+		this.exponents = Collections.unmodifiableMap(ConditionalExistenceCollections.conditionalExistenceMap(exponents,
+				e -> !Integer.valueOf(0).equals(e.getValue())));
+	}
+
+	/**
+	 * Calculates the quotient of two products
+	 *
+	 * @param other
+	 *            other product
+	 * @return quotient of two products
+	 * @since 2019-10-16
+	 * @throws NullPointerException
+	 *             if other is null
+	 */
+	public ObjectProduct<T> dividedBy(final ObjectProduct<T> other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		// get a list of all objects in both sets
+		final Set<T> objects = new HashSet<>();
+		objects.addAll(this.getBaseSet());
+		objects.addAll(other.getBaseSet());
+
+		// get a list of all exponents
+		final Map<T, Integer> map = new HashMap<>(objects.size());
+		for (final T key : objects) {
+			map.put(key, this.getExponent(key) - other.getExponent(key));
+		}
+
+		// create the product
+		return new ObjectProduct<>(map);
+	}
+
+	// this method relies on the use of ZeroIsNullMap
+	@Override
+	public boolean equals(final Object obj) {
+		if (this == obj)
+			return true;
+		if (!(obj instanceof ObjectProduct))
+			return false;
+		final ObjectProduct<?> other = (ObjectProduct<?>) obj;
+		return Objects.equals(this.exponents, other.exponents);
+	}
+
+	/**
+	 * @return immutable map mapping objects to exponents
+	 * @since 2019-10-16
+	 */
+	public Map<T, Integer> exponentMap() {
+		return this.exponents;
+	}
+
+	/**
+	 * @return a set of all of the base objects with non-zero exponents that make up this dimension.
+	 * @since 2018-12-12
+	 * @since v0.1.0
+	 */
+	public final Set<T> getBaseSet() {
+		final Set<T> dimensions = new HashSet<>();
+
+		// add all dimensions with a nonzero exponent - zero exponents shouldn't be there in the first place
+		for (final T dimension : this.exponents.keySet()) {
+			if (!this.exponents.get(dimension).equals(0)) {
+				dimensions.add(dimension);
+			}
+		}
+
+		return dimensions;
+	}
+
+	/**
+	 * Gets the exponent for a specific dimension.
+	 * 
+	 * @param dimension
+	 *            dimension to check
+	 * @return exponent for that dimension
+	 * @since 2018-12-12
+	 * @since v0.1.0
+	 */
+	public int getExponent(final T dimension) {
+		return this.exponents.getOrDefault(dimension, 0);
+	}
+
+	@Override
+	public int hashCode() {
+		return Objects.hash(this.exponents);
+	}
+
+	/**
+	 * @return true if this product is a single object, i.e. it has one exponent of one and no other nonzero exponents
+	 * @since 2019-10-16
+	 */
+	public boolean isSingleObject() {
+		int oneCount = 0;
+		boolean twoOrMore = false; // has exponents of 2 or more
+		for (final T b : this.getBaseSet()) {
+			if (this.getExponent(b) == 1) {
+				oneCount++;
+			} else if (this.getExponent(b) != 0) {
+				twoOrMore = true;
+			}
+		}
+		return oneCount == 1 && !twoOrMore;
+	}
+
+	/**
+	 * Multiplies this product by another
+	 *
+	 * @param other
+	 *            other product
+	 * @return product of two products
+	 * @since 2019-10-16
+	 * @throws NullPointerException
+	 *             if other is null
+	 */
+	public ObjectProduct<T> times(final ObjectProduct<T> other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		// get a list of all objects in both sets
+		final Set<T> objects = new HashSet<>();
+		objects.addAll(this.getBaseSet());
+		objects.addAll(other.getBaseSet());
+
+		// get a list of all exponents
+		final Map<T, Integer> map = new HashMap<>(objects.size());
+		for (final T key : objects) {
+			map.put(key, this.getExponent(key) + other.getExponent(key));
+		}
+
+		// create the product
+		return new ObjectProduct<>(map);
+	}
+
+	/**
+	 * Returns this product, but to an exponent
+	 * 
+	 * @param exponent
+	 *            exponent
+	 * @return result of exponentiation
+	 * @since 2019-10-16
+	 */
+	public ObjectProduct<T> toExponent(final int exponent) {
+		final Map<T, Integer> map = new HashMap<>(this.exponents);
+		for (final T key : this.exponents.keySet()) {
+			map.put(key, this.getExponent(key) * exponent);
+		}
+		return new ObjectProduct<>(map);
+	}
+
+	/**
+	 * Converts this product to a string using the objects' {@link Object#toString()} method. If objects have a long
+	 * toString representation, it is recommended to use {@link #toString(Function)} instead to shorten the returned
+	 * string.
+	 * 
+	 * <p>
+	 * {@inheritDoc}
+	 */
+	@Override
+	public String toString() {
+		return this.toString(Object::toString);
+	}
+
+	/**
+	 * Converts this product to a string. The objects that make up this product are represented by
+	 * {@code objectToString}
+	 * 
+	 * @param objectToString
+	 *            function to convert objects to strings
+	 * @return string representation of product
+	 * @since 2019-10-16
+	 */
+	public String toString(final Function<T, String> objectToString) {
+		final List<String> positiveStringComponents = new ArrayList<>();
+		final List<String> negativeStringComponents = new ArrayList<>();
+
+		// for each base object that makes up this object, add it and its exponent
+		for (final T object : this.getBaseSet()) {
+			final int exponent = this.exponents.get(object);
+			if (exponent > 0) {
+				positiveStringComponents.add(String.format("%s^%d", objectToString.apply(object), exponent));
+			} else if (exponent < 0) {
+				negativeStringComponents.add(String.format("%s^%d", objectToString.apply(object), -exponent));
+			}
+		}
+
+		final String positiveString = positiveStringComponents.isEmpty() ? "1"
+				: String.join(" * ", positiveStringComponents);
+		final String negativeString = negativeStringComponents.isEmpty() ? ""
+				: " / " + String.join(" * ", negativeStringComponents);
+
+		return positiveString + negativeString;
+	}
+}
diff --git a/src/main/java/org/unitConverter/math/UncertainDouble.java b/src/main/java/org/unitConverter/math/UncertainDouble.java
new file mode 100644
index 0000000..3651bd5
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/UncertainDouble.java
@@ -0,0 +1,419 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.math;
+
+import java.math.BigDecimal;
+import java.math.RoundingMode;
+import java.util.Objects;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+/**
+ * A double with an associated uncertainty value. For example, 3.2 ± 0.2.
+ * <p>
+ * All methods in this class throw a NullPointerException if any of their
+ * arguments is null.
+ *
+ * @since 2020-09-07
+ */
+public final class UncertainDouble implements Comparable<UncertainDouble> {
+	/**
+	 * The exact value 0
+	 */
+	public static final UncertainDouble ZERO = UncertainDouble.of(0, 0);
+	
+	/**
+	 * A regular expression that can recognize toString forms
+	 */
+	private static final Pattern TO_STRING = Pattern
+			.compile("([a-zA-Z_0-9\\.\\,]+)" // a number
+					// optional "± [number]"
+					+ "(?:\\s*(?:±|\\+-)\\s*([a-zA-Z_0-9\\.\\,]+))?");
+	
+	/**
+	 * Parses a string in the form of {@link UncertainDouble#toString(boolean)}
+	 * and returns the corresponding {@code UncertainDouble} instance.
+	 * <p>
+	 * This method allows some alternative forms of the string representation,
+	 * such as using "+-" instead of "±".
+	 * 
+	 * @param s string to parse
+	 * @return {@code UncertainDouble} instance
+	 * @throws IllegalArgumentException if the string is invalid
+	 * @since 2020-09-07
+	 */
+	public static final UncertainDouble fromString(String s) {
+		Objects.requireNonNull(s, "s may not be null");
+		final Matcher matcher = TO_STRING.matcher(s);
+		
+		double value, uncertainty;
+		try {
+			value = Double.parseDouble(matcher.group(1));
+		} catch (IllegalStateException | NumberFormatException e) {
+			throw new IllegalArgumentException(
+					"String " + s + " not in correct format.");
+		}
+		
+		final String uncertaintyString = matcher.group(2);
+		if (uncertaintyString == null) {
+			uncertainty = 0;
+		} else {
+			try {
+				uncertainty = Double.parseDouble(uncertaintyString);
+			} catch (final NumberFormatException e) {
+				throw new IllegalArgumentException(
+						"String " + s + " not in correct format.");
+			}
+		}
+		
+		return UncertainDouble.of(value, uncertainty);
+	}
+	
+	/**
+	 * Gets an {@code UncertainDouble} from its value and <b>absolute</b>
+	 * uncertainty.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public static final UncertainDouble of(double value, double uncertainty) {
+		return new UncertainDouble(value, uncertainty);
+	}
+	
+	/**
+	 * Gets an {@code UncertainDouble} from its value and <b>relative</b>
+	 * uncertainty.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public static final UncertainDouble ofRelative(double value,
+			double relativeUncertainty) {
+		return new UncertainDouble(value, value * relativeUncertainty);
+	}
+	
+	private final double value;
+	
+	private final double uncertainty;
+	
+	/**
+	 * @param value
+	 * @param uncertainty
+	 * @since 2020-09-07
+	 */
+	private UncertainDouble(double value, double uncertainty) {
+		this.value = value;
+		// uncertainty should only ever be positive
+		this.uncertainty = Math.abs(uncertainty);
+	}
+	
+	/**
+	 * Compares this {@code UncertainDouble} with another
+	 * {@code UncertainDouble}.
+	 * <p>
+	 * This method only compares the values, not the uncertainties. So 3.1 ± 0.5
+	 * is considered less than 3.2 ± 0.5, even though they are equivalent.
+	 * <p>
+	 * <b>Note:</b> The natural ordering of this class is inconsistent with
+	 * equals. Specifically, if two {@code UncertainDouble} instances {@code a}
+	 * and {@code b} have the same value but different uncertainties,
+	 * {@code a.compareTo(b)} will return 0 but {@code a.equals(b)} will return
+	 * {@code false}.
+	 */
+	@Override
+	public final int compareTo(UncertainDouble o) {
+		return Double.compare(this.value, o.value);
+	}
+	
+	/**
+	 * Returns the quotient of {@code this} and {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble dividedBy(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		return UncertainDouble.ofRelative(this.value / other.value, Math
+				.hypot(this.relativeUncertainty(), other.relativeUncertainty()));
+	}
+	
+	/**
+	 * Returns the quotient of {@code this} and the exact value {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble dividedByExact(double other) {
+		return UncertainDouble.of(this.value / other, this.uncertainty / other);
+	}
+	
+	@Override
+	public final boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (!(obj instanceof UncertainDouble))
+			return false;
+		final UncertainDouble other = (UncertainDouble) obj;
+		if (Double.compare(this.value, other.value) != 0)
+			return false;
+		if (Double.compare(this.uncertainty, other.uncertainty) != 0)
+			return false;
+		return true;
+	}
+	
+	/**
+	 * @param other another {@code UncertainDouble}
+	 * @return true iff this and {@code other} are within each other's
+	 *         uncertainty range.
+	 * @since 2020-09-07
+	 */
+	public final boolean equivalent(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		return Math.abs(this.value - other.value) <= Math.min(this.uncertainty,
+				other.uncertainty);
+	}
+	
+	/**
+	 * Gets the preferred scale for rounding a value for toString.
+	 * 
+	 * @since 2020-09-07
+	 */
+	private final int getDisplayScale() {
+		// round based on uncertainty
+		// if uncertainty starts with 1 (ignoring zeroes and the decimal
+		// point), rounds
+		// so that uncertainty has 2 significant digits.
+		// otherwise, rounds so that uncertainty has 1 significant digits.
+		// the value is rounded to the same number of decimal places as the
+		// uncertainty.
+		final BigDecimal bigUncertainty = BigDecimal.valueOf(this.uncertainty);
+		
+		// the scale that will give the uncertainty two decimal places
+		final int twoDecimalPlacesScale = bigUncertainty.scale()
+				- bigUncertainty.precision() + 2;
+		final BigDecimal roundedUncertainty = bigUncertainty
+				.setScale(twoDecimalPlacesScale, RoundingMode.HALF_EVEN);
+		
+		if (roundedUncertainty.unscaledValue().intValue() >= 20)
+			return twoDecimalPlacesScale - 1; // one decimal place
+		else
+			return twoDecimalPlacesScale;
+	}
+	
+	@Override
+	public final int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result + Double.hashCode(this.value);
+		result = prime * result + Double.hashCode(this.uncertainty);
+		return result;
+	}
+	
+	/**
+	 * @return true iff the value has no uncertainty
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final boolean isExact() {
+		return this.uncertainty == 0;
+	}
+	
+	/**
+	 * Returns the difference of {@code this} and {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble minus(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		return UncertainDouble.of(this.value - other.value,
+				Math.hypot(this.uncertainty, other.uncertainty));
+	}
+	
+	/**
+	 * Returns the difference of {@code this} and the exact value {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble minusExact(double other) {
+		return UncertainDouble.of(this.value - other, this.uncertainty);
+	}
+	
+	/**
+	 * Returns the sum of {@code this} and {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble plus(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		return UncertainDouble.of(this.value + other.value,
+				Math.hypot(this.uncertainty, other.uncertainty));
+	}
+	
+	/**
+	 * Returns the sum of {@code this} and the exact value {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble plusExact(double other) {
+		return UncertainDouble.of(this.value + other, this.uncertainty);
+	}
+	
+	/**
+	 * @return relative uncertainty
+	 * @since 2020-09-07
+	 */
+	public final double relativeUncertainty() {
+		return this.uncertainty / this.value;
+	}
+	
+	/**
+	 * Returns the product of {@code this} and {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble times(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		return UncertainDouble.ofRelative(this.value * other.value, Math
+				.hypot(this.relativeUncertainty(), other.relativeUncertainty()));
+	}
+	
+	/**
+	 * Returns the product of {@code this} and the exact value {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble timesExact(double other) {
+		return UncertainDouble.of(this.value * other, this.uncertainty * other);
+	}
+	
+	/**
+	 * Returns the result of {@code this} raised to the exponent {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble toExponent(UncertainDouble other) {
+		Objects.requireNonNull(other, "other may not be null");
+		
+		final double result = Math.pow(this.value, other.value);
+		final double relativeUncertainty = Math.hypot(
+				other.value * this.relativeUncertainty(),
+				Math.log(this.value) * other.uncertainty);
+		
+		return UncertainDouble.ofRelative(result, relativeUncertainty);
+	}
+	
+	/**
+	 * Returns the result of {@code this} raised the exact exponent
+	 * {@code other}.
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final UncertainDouble toExponentExact(double other) {
+		return UncertainDouble.ofRelative(Math.pow(this.value, other),
+				this.relativeUncertainty() * other);
+	}
+	
+	/**
+	 * Returns a string representation of this {@code UncertainDouble}.
+	 * <p>
+	 * This method returns the same value as {@link #toString(boolean)}, but
+	 * {@code showUncertainty} is true if and only if the uncertainty is
+	 * non-zero.
+	 * 
+	 * <p>
+	 * Examples:
+	 * 
+	 * <pre>
+	 * UncertainDouble.of(3.27, 0.22).toString() = "3.3 ± 0.2"
+	 * UncertainDouble.of(3.27, 0.13).toString() = "3.27 ± 0.13"
+	 * UncertainDouble.of(-5.01, 0).toString() = "-5.01"
+	 * </pre>
+	 * 
+	 * @since 2020-09-07
+	 */
+	@Override
+	public final String toString() {
+		return this.toString(!this.isExact());
+	}
+	
+	/**
+	 * Returns a string representation of this {@code UncertainDouble}.
+	 * <p>
+	 * If {@code showUncertainty} is true, the string will be of the form "VALUE
+	 * ± UNCERTAINTY", and if it is false the string will be of the form "VALUE"
+	 * <p>
+	 * VALUE represents a string representation of this {@code UncertainDouble}'s
+	 * value. If the uncertainty is non-zero, the string will be rounded to the
+	 * same precision as the uncertainty, otherwise it will not be rounded. The
+	 * string is still rounded if {@code showUncertainty} is false.<br>
+	 * UNCERTAINTY represents a string representation of this
+	 * {@code UncertainDouble}'s uncertainty. If the uncertainty ends in 1X
+	 * (where X represents any digit) it will be rounded to two significant
+	 * digits otherwise it will be rounded to one significant digit.
+	 * <p>
+	 * Examples:
+	 * 
+	 * <pre>
+	 * UncertainDouble.of(3.27, 0.22).toString(false) = "3.3"
+	 * UncertainDouble.of(3.27, 0.22).toString(true) = "3.3 ± 0.2"
+	 * UncertainDouble.of(3.27, 0.13).toString(false) = "3.27"
+	 * UncertainDouble.of(3.27, 0.13).toString(true) = "3.27 ± 0.13"
+	 * UncertainDouble.of(-5.01, 0).toString(false) = "-5.01"
+	 * UncertainDouble.of(-5.01, 0).toString(true) = "-5.01 ± 0.0"
+	 * </pre>
+	 * 
+	 * @since 2020-09-07
+	 */
+	public final String toString(boolean showUncertainty) {
+		String valueString, uncertaintyString;
+		
+		// generate the string representation of value and uncertainty
+		if (this.isExact()) {
+			uncertaintyString = "0.0";
+			valueString = Double.toString(this.value);
+			
+		} else {
+			// round the value and uncertainty according to getDisplayScale()
+			final BigDecimal bigValue = BigDecimal.valueOf(this.value);
+			final BigDecimal bigUncertainty = BigDecimal.valueOf(this.uncertainty);
+			
+			final int displayScale = this.getDisplayScale();
+			final BigDecimal roundedUncertainty = bigUncertainty
+					.setScale(displayScale, RoundingMode.HALF_EVEN);
+			final BigDecimal roundedValue = bigValue.setScale(displayScale,
+					RoundingMode.HALF_EVEN);
+			
+			valueString = roundedValue.toString();
+			uncertaintyString = roundedUncertainty.toString();
+		}
+		
+		// return "value" or "value ± uncertainty" depending on showUncertainty
+		return valueString + (showUncertainty ? " ± " + uncertaintyString : "");
+	}
+	
+	/**
+	 * @return absolute uncertainty
+	 * @since 2020-09-07
+	 */
+	public final double uncertainty() {
+		return this.uncertainty;
+	}
+	
+	/**
+	 * @return value without uncertainty
+	 * @since 2020-09-07
+	 */
+	public final double value() {
+		return this.value;
+	}
+}
diff --git a/src/main/java/org/unitConverter/math/package-info.java b/src/main/java/org/unitConverter/math/package-info.java
new file mode 100644
index 0000000..65727e4
--- /dev/null
+++ b/src/main/java/org/unitConverter/math/package-info.java
@@ -0,0 +1,24 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+/**
+ * Supplementary classes that are not related to units, but are necessary for their function.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-03-14
+ * @since v0.2.0
+ */
+package org.unitConverter.math;
\ No newline at end of file
diff --git a/src/main/java/org/unitConverter/package-info.java b/src/main/java/org/unitConverter/package-info.java
new file mode 100644
index 0000000..23dd165
--- /dev/null
+++ b/src/main/java/org/unitConverter/package-info.java
@@ -0,0 +1,24 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+/**
+ * A program that converts units.
+ * 
+ * @author Adrien Hopkins
+ * @version v0.2.0
+ * @since 2019-01-25
+ */
+package org.unitConverter;
\ No newline at end of file
diff --git a/src/main/java/org/unitConverter/unit/BaseDimension.java b/src/main/java/org/unitConverter/unit/BaseDimension.java
new file mode 100644
index 0000000..8e63a17
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/BaseDimension.java
@@ -0,0 +1,87 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+
+/**
+ * A dimension that defines a {@code BaseUnit}
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class BaseDimension {
+	/**
+	 * Gets a {@code BaseDimension} with the provided name and symbol.
+	 * 
+	 * @param name
+	 *            name of dimension
+	 * @param symbol
+	 *            symbol used for dimension
+	 * @return dimension
+	 * @since 2019-10-16
+	 */
+	public static BaseDimension valueOf(final String name, final String symbol) {
+		return new BaseDimension(name, symbol);
+	}
+
+	/**
+	 * The name of the dimension.
+	 */
+	private final String name;
+	/**
+	 * The symbol used by the dimension. Symbols should be short, generally one or two characters.
+	 */
+	private final String symbol;
+
+	/**
+	 * Creates the {@code BaseDimension}.
+	 * 
+	 * @param name
+	 *            name of unit
+	 * @param symbol
+	 *            symbol of unit
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-10-16
+	 */
+	private BaseDimension(final String name, final String symbol) {
+		this.name = Objects.requireNonNull(name, "name must not be null.");
+		this.symbol = Objects.requireNonNull(symbol, "symbol must not be null.");
+	}
+
+	/**
+	 * @return name
+	 * @since 2019-10-16
+	 */
+	public final String getName() {
+		return this.name;
+	}
+
+	/**
+	 * @return symbol
+	 * @since 2019-10-16
+	 */
+	public final String getSymbol() {
+		return this.symbol;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("%s (%s)", this.getName(), this.getSymbol());
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/BaseUnit.java b/src/main/java/org/unitConverter/unit/BaseUnit.java
new file mode 100644
index 0000000..6757bd0
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/BaseUnit.java
@@ -0,0 +1,133 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.HashSet;
+import java.util.Objects;
+import java.util.Set;
+
+/**
+ * A unit that other units are defined by.
+ * <p>
+ * Note that BaseUnits <b>must</b> have names and symbols. This is because they
+ * are used for toString code. Therefore, the Optionals provided by
+ * {@link #getPrimaryName} and {@link #getSymbol} will always contain a value.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class BaseUnit extends Unit {
+	/**
+	 * Gets a base unit from the dimension it measures, its name and its symbol.
+	 * 
+	 * @param dimension dimension measured by this unit
+	 * @param name      name of unit
+	 * @param symbol    symbol of unit
+	 * @return base unit
+	 * @since 2019-10-16
+	 */
+	public static BaseUnit valueOf(final BaseDimension dimension,
+			final String name, final String symbol) {
+		return new BaseUnit(dimension, name, symbol, new HashSet<>());
+	}
+	
+	/**
+	 * Gets a base unit from the dimension it measures, its name and its symbol.
+	 * 
+	 * @param dimension dimension measured by this unit
+	 * @param name      name of unit
+	 * @param symbol    symbol of unit
+	 * @return base unit
+	 * @since 2019-10-21
+	 */
+	public static BaseUnit valueOf(final BaseDimension dimension,
+			final String name, final String symbol, final Set<String> otherNames) {
+		return new BaseUnit(dimension, name, symbol, otherNames);
+	}
+	
+	/**
+	 * The dimension measured by this base unit.
+	 */
+	private final BaseDimension dimension;
+	
+	/**
+	 * Creates the {@code BaseUnit}.
+	 * 
+	 * @param dimension   dimension of unit
+	 * @param primaryName name of unit
+	 * @param symbol      symbol of unit
+	 * @throws NullPointerException if any argument is null
+	 * @since 2019-10-16
+	 */
+	private BaseUnit(final BaseDimension dimension, final String primaryName,
+			final String symbol, final Set<String> otherNames) {
+		super(primaryName, symbol, otherNames);
+		this.dimension = Objects.requireNonNull(dimension,
+				"dimension must not be null.");
+	}
+	
+	/**
+	 * Returns a {@code LinearUnit} with this unit as a base and a conversion
+	 * factor of 1. This operation must be done in order to allow units to be
+	 * created with operations.
+	 * 
+	 * @return this unit as a {@code LinearUnit}
+	 * @since 2019-10-16
+	 */
+	public LinearUnit asLinearUnit() {
+		return LinearUnit.valueOf(this.getBase(), 1);
+	}
+	
+	@Override
+	protected double convertFromBase(final double value) {
+		return value;
+	}
+	
+	@Override
+	protected double convertToBase(final double value) {
+		return value;
+	}
+	
+	/**
+	 * @return dimension
+	 * @since 2019-10-16
+	 */
+	public final BaseDimension getBaseDimension() {
+		return this.dimension;
+	}
+	
+	@Override
+	public String toString() {
+		return this.getPrimaryName().orElse("Unnamed unit")
+				+ (this.getSymbol().isPresent()
+						? String.format(" (%s)", this.getSymbol().get())
+						: "");
+	}
+	
+	@Override
+	public BaseUnit withName(final NameSymbol ns) {
+		Objects.requireNonNull(ns, "ns must not be null.");
+		if (!ns.getPrimaryName().isPresent())
+			throw new IllegalArgumentException(
+					"BaseUnits must have primary names.");
+		if (!ns.getSymbol().isPresent())
+			throw new IllegalArgumentException("BaseUnits must have symbols.");
+		return BaseUnit.valueOf(this.getBaseDimension(),
+				ns.getPrimaryName().get(), ns.getSymbol().get(),
+				ns.getOtherNames());
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/BritishImperial.java b/src/main/java/org/unitConverter/unit/BritishImperial.java
new file mode 100644
index 0000000..ea23cd1
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/BritishImperial.java
@@ -0,0 +1,116 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+/**
+ * A static utility class that contains units in the British Imperial system.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-21
+ */
+public final class BritishImperial {
+	/**
+	 * Imperial units that measure area
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Area {
+		public static final LinearUnit SQUARE_FOOT = Length.FOOT.toExponent(2);
+		public static final LinearUnit SQUARE_YARD = Length.YARD.toExponent(2);
+		public static final LinearUnit SQUARE_MILE = Length.MILE.toExponent(2);
+		public static final LinearUnit PERCH = Length.ROD.times(Length.ROD);
+		public static final LinearUnit ROOD = Length.ROD.times(Length.FURLONG);
+		public static final LinearUnit ACRE = Length.FURLONG.times(Length.CHAIN);
+	}
+
+	/**
+	 * Imperial units that measure length
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-28
+	 */
+	public static final class Length {
+		/**
+		 * According to the International Yard and Pound of 1959, a yard is defined as exactly 0.9144 metres.
+		 */
+		public static final LinearUnit YARD = SI.METRE.times(0.9144);
+		public static final LinearUnit FOOT = YARD.dividedBy(3);
+		public static final LinearUnit INCH = FOOT.dividedBy(12);
+		public static final LinearUnit THOU = INCH.dividedBy(1000);
+		public static final LinearUnit CHAIN = YARD.times(22);
+		public static final LinearUnit FURLONG = CHAIN.times(10);
+		public static final LinearUnit MILE = FURLONG.times(8);
+		public static final LinearUnit LEAGUE = MILE.times(3);
+
+		public static final LinearUnit NAUTICAL_MILE = SI.METRE.times(1852);
+		public static final LinearUnit CABLE = NAUTICAL_MILE.dividedBy(10);
+		public static final LinearUnit FATHOM = CABLE.dividedBy(100);
+
+		public static final LinearUnit ROD = YARD.times(5.5);
+		public static final LinearUnit LINK = ROD.dividedBy(25);
+	}
+
+	/**
+	 * British Imperial units that measure mass.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Mass {
+		public static final LinearUnit POUND = SI.GRAM.times(453.59237);
+		public static final LinearUnit OUNCE = POUND.dividedBy(16);
+		public static final LinearUnit DRACHM = POUND.dividedBy(256);
+		public static final LinearUnit GRAIN = POUND.dividedBy(7000);
+		public static final LinearUnit STONE = POUND.times(14);
+		public static final LinearUnit QUARTER = STONE.times(2);
+		public static final LinearUnit HUNDREDWEIGHT = QUARTER.times(4);
+		public static final LinearUnit LONG_TON = HUNDREDWEIGHT.times(20);
+		public static final LinearUnit SLUG = SI.KILOGRAM.times(14.59390294);
+	}
+
+	/**
+	 * British Imperial units that measure volume
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Volume {
+		public static final LinearUnit FLUID_OUNCE = SI.LITRE.withPrefix(SI.MILLI).times(28.4130625);
+		public static final LinearUnit GILL = FLUID_OUNCE.times(5);
+		public static final LinearUnit PINT = FLUID_OUNCE.times(20);
+		public static final LinearUnit QUART = PINT.times(2);
+		public static final LinearUnit GALLON = QUART.times(4);
+		public static final LinearUnit PECK = GALLON.times(2);
+		public static final LinearUnit BUSHEL = PECK.times(4);
+
+		public static final LinearUnit CUBIC_INCH = Length.INCH.toExponent(3);
+		public static final LinearUnit CUBIC_FOOT = Length.FOOT.toExponent(3);
+		public static final LinearUnit CUBIC_YARD = Length.YARD.toExponent(3);
+		public static final LinearUnit ACRE_FOOT = Area.ACRE.times(Length.FOOT);
+	}
+
+	public static final LinearUnit OUNCE_FORCE = Mass.OUNCE.times(SI.Constants.EARTH_GRAVITY);
+	public static final LinearUnit POUND_FORCE = Mass.POUND.times(SI.Constants.EARTH_GRAVITY);
+
+	public static final LinearUnit BRITISH_THERMAL_UNIT = SI.JOULE.times(1055.06);
+	public static final LinearUnit CALORIE = SI.JOULE.times(4.184);
+	public static final LinearUnit KILOCALORIE = SI.JOULE.times(4184);
+
+	public static final Unit FAHRENHEIT = Unit.fromConversionFunctions(SI.KELVIN.getBase(),
+			tempK -> tempK * 1.8 - 459.67, tempF -> (tempF + 459.67) / 1.8);
+}
diff --git a/src/main/java/org/unitConverter/unit/FunctionalUnit.java b/src/main/java/org/unitConverter/unit/FunctionalUnit.java
new file mode 100644
index 0000000..586e0d7
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/FunctionalUnit.java
@@ -0,0 +1,109 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+import java.util.function.DoubleUnaryOperator;
+
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * A unit that uses functional objects to convert to and from its base.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-05-22
+ */
+final class FunctionalUnit extends Unit {
+	/**
+	 * A function that accepts a value expressed in the unit's base and returns that value expressed in this unit.
+	 * 
+	 * @since 2019-05-22
+	 */
+	private final DoubleUnaryOperator converterFrom;
+
+	/**
+	 * A function that accepts a value expressed in the unit and returns that value expressed in the unit's base.
+	 * 
+	 * @since 2019-05-22
+	 */
+	private final DoubleUnaryOperator converterTo;
+
+	/**
+	 * Creates the {@code FunctionalUnit}.
+	 * 
+	 * @param base
+	 *            unit's base
+	 * @param converterFrom
+	 *            function that accepts a value expressed in the unit's base and returns that value expressed in this
+	 *            unit.
+	 * @param converterTo
+	 *            function that accepts a value expressed in the unit and returns that value expressed in the unit's
+	 *            base.
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-05-22
+	 */
+	public FunctionalUnit(final ObjectProduct<BaseUnit> base, final DoubleUnaryOperator converterFrom,
+			final DoubleUnaryOperator converterTo) {
+		super(base, NameSymbol.EMPTY);
+		this.converterFrom = Objects.requireNonNull(converterFrom, "converterFrom must not be null.");
+		this.converterTo = Objects.requireNonNull(converterTo, "converterTo must not be null.");
+	}
+
+	/**
+	 * Creates the {@code FunctionalUnit}.
+	 * 
+	 * @param base
+	 *            unit's base
+	 * @param converterFrom
+	 *            function that accepts a value expressed in the unit's base and returns that value expressed in this
+	 *            unit.
+	 * @param converterTo
+	 *            function that accepts a value expressed in the unit and returns that value expressed in the unit's
+	 *            base.
+	 * @throws NullPointerException
+	 *             if any argument is null
+	 * @since 2019-05-22
+	 */
+	public FunctionalUnit(final ObjectProduct<BaseUnit> base, final DoubleUnaryOperator converterFrom,
+			final DoubleUnaryOperator converterTo, final NameSymbol ns) {
+		super(base, ns);
+		this.converterFrom = Objects.requireNonNull(converterFrom, "converterFrom must not be null.");
+		this.converterTo = Objects.requireNonNull(converterTo, "converterTo must not be null.");
+	}
+
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses {@code converterFrom} to convert.
+	 */
+	@Override
+	public double convertFromBase(final double value) {
+		return this.converterFrom.applyAsDouble(value);
+	}
+
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses {@code converterTo} to convert.
+	 */
+	@Override
+	public double convertToBase(final double value) {
+		return this.converterTo.applyAsDouble(value);
+	}
+
+}
diff --git a/src/main/java/org/unitConverter/unit/FunctionalUnitlike.java b/src/main/java/org/unitConverter/unit/FunctionalUnitlike.java
new file mode 100644
index 0000000..21c1fca
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/FunctionalUnitlike.java
@@ -0,0 +1,72 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.function.DoubleFunction;
+import java.util.function.ToDoubleFunction;
+
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * A unitlike form that converts using two conversion functions.
+ *
+ * @since 2020-09-07
+ */
+final class FunctionalUnitlike<V> extends Unitlike<V> {
+	/**
+	 * A function that accepts a value in the unitlike form's base and returns a
+	 * value in the unitlike form.
+	 * 
+	 * @since 2020-09-07
+	 */
+	private final DoubleFunction<V> converterFrom;
+	
+	/**
+	 * A function that accepts a value in the unitlike form and returns a value
+	 * in the unitlike form's base.
+	 */
+	private final ToDoubleFunction<V> converterTo;
+	
+	/**
+	 * Creates the {@code FunctionalUnitlike}.
+	 * 
+	 * @param base          unitlike form's base
+	 * @param converterFrom function that accepts a value in the unitlike form's
+	 *                      base and returns a value in the unitlike form.
+	 * @param converterTo   function that accepts a value in the unitlike form
+	 *                      and returns a value in the unitlike form's base.
+	 * @throws NullPointerException if any argument is null
+	 * @since 2019-05-22
+	 */
+	protected FunctionalUnitlike(ObjectProduct<BaseUnit> unitBase, NameSymbol ns,
+			DoubleFunction<V> converterFrom, ToDoubleFunction<V> converterTo) {
+		super(unitBase, ns);
+		this.converterFrom = converterFrom;
+		this.converterTo = converterTo;
+	}
+	
+	@Override
+	protected V convertFromBase(double value) {
+		return this.converterFrom.apply(value);
+	}
+	
+	@Override
+	protected double convertToBase(V value) {
+		return this.converterTo.applyAsDouble(value);
+	}
+	
+}
diff --git a/src/main/java/org/unitConverter/unit/LinearUnit.java b/src/main/java/org/unitConverter/unit/LinearUnit.java
new file mode 100644
index 0000000..b7f33d5
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/LinearUnit.java
@@ -0,0 +1,441 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+
+import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.ObjectProduct;
+import org.unitConverter.math.UncertainDouble;
+
+/**
+ * A unit that can be expressed as a product of its base and a number. For
+ * example, kilometres, inches and pounds.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class LinearUnit extends Unit {
+	/**
+	 * Gets a {@code LinearUnit} from a unit and a value. For example, converts
+	 * '59 °F' to a linear unit with the value of '288.15 K'
+	 * 
+	 * @param unit  unit to convert
+	 * @param value value to convert
+	 * @return value expressed as a {@code LinearUnit}
+	 * @since 2019-10-16
+	 * @throws NullPointerException if unit is null
+	 */
+	public static LinearUnit fromUnitValue(final Unit unit, final double value) {
+		return new LinearUnit(
+				Objects.requireNonNull(unit, "unit must not be null.").getBase(),
+				unit.convertToBase(value), NameSymbol.EMPTY);
+	}
+	
+	/**
+	 * Gets a {@code LinearUnit} from a unit and a value. For example, converts
+	 * '59 °F' to a linear unit with the value of '288.15 K'
+	 * 
+	 * @param unit  unit to convert
+	 * @param value value to convert
+	 * @param ns    name(s) and symbol of unit
+	 * @return value expressed as a {@code LinearUnit}
+	 * @since 2019-10-21
+	 * @throws NullPointerException if unit or ns is null
+	 */
+	public static LinearUnit fromUnitValue(final Unit unit, final double value,
+			final NameSymbol ns) {
+		return new LinearUnit(
+				Objects.requireNonNull(unit, "unit must not be null.").getBase(),
+				unit.convertToBase(value), ns);
+	}
+	
+	/**
+	 * @return the base unit associated with {@code unit}, as a
+	 *         {@code LinearUnit}.
+	 * @since 2020-10-02
+	 */
+	public static LinearUnit getBase(final Unit unit) {
+		return new LinearUnit(unit.getBase(), 1, NameSymbol.EMPTY);
+	}
+	
+	/**
+	 * @return the base unit associated with {@code unitlike}, as a
+	 *         {@code LinearUnit}.
+	 * @since 2020-10-02
+	 */
+	public static LinearUnit getBase(final Unitlike<?> unit) {
+		return new LinearUnit(unit.getBase(), 1, NameSymbol.EMPTY);
+	}
+	
+	/**
+	 * Gets a {@code LinearUnit} from a unit base and a conversion factor. In
+	 * other words, gets the product of {@code unitBase} and
+	 * {@code conversionFactor}, expressed as a {@code LinearUnit}.
+	 * 
+	 * @param unitBase         unit base to multiply by
+	 * @param conversionFactor number to multiply base by
+	 * @return product of base and conversion factor
+	 * @since 2019-10-16
+	 * @throws NullPointerException if unitBase is null
+	 */
+	public static LinearUnit valueOf(final ObjectProduct<BaseUnit> unitBase,
+			final double conversionFactor) {
+		return new LinearUnit(unitBase, conversionFactor, NameSymbol.EMPTY);
+	}
+	
+	/**
+	 * Gets a {@code LinearUnit} from a unit base and a conversion factor. In
+	 * other words, gets the product of {@code unitBase} and
+	 * {@code conversionFactor}, expressed as a {@code LinearUnit}.
+	 * 
+	 * @param unitBase         unit base to multiply by
+	 * @param conversionFactor number to multiply base by
+	 * @param ns               name(s) and symbol of unit
+	 * @return product of base and conversion factor
+	 * @since 2019-10-21
+	 * @throws NullPointerException if unitBase is null
+	 */
+	public static LinearUnit valueOf(final ObjectProduct<BaseUnit> unitBase,
+			final double conversionFactor, final NameSymbol ns) {
+		return new LinearUnit(unitBase, conversionFactor, ns);
+	}
+	
+	/**
+	 * The value of this unit as represented in its base form. Mathematically,
+	 * 
+	 * <pre>
+	 * this = conversionFactor * getBase()
+	 * </pre>
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final double conversionFactor;
+	
+	/**
+	 * Creates the {@code LinearUnit}.
+	 * 
+	 * @param unitBase         base of linear unit
+	 * @param conversionFactor conversion factor between base and unit
+	 * @since 2019-10-16
+	 */
+	private LinearUnit(final ObjectProduct<BaseUnit> unitBase,
+			final double conversionFactor, final NameSymbol ns) {
+		super(unitBase, ns);
+		this.conversionFactor = conversionFactor;
+	}
+	
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Converts by dividing by {@code conversionFactor}
+	 */
+	@Override
+	protected double convertFromBase(final double value) {
+		return value / this.getConversionFactor();
+	}
+	
+	/**
+	 * Converts an {@code UncertainDouble} value expressed in this unit to an
+	 * {@code UncertainValue} value expressed in {@code other}.
+	 * 
+	 * @param other unit to convert to
+	 * @param value value to convert
+	 * @return converted value
+	 * @since 2019-09-07
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unit (as tested by
+	 *                                  {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if value or other is null
+	 */
+	public UncertainDouble convertTo(LinearUnit other, UncertainDouble value) {
+		Objects.requireNonNull(other, "other must not be null.");
+		Objects.requireNonNull(value, "value may not be null.");
+		if (this.canConvertTo(other))
+			return value.timesExact(
+					this.getConversionFactor() / other.getConversionFactor());
+		else
+			throw new IllegalArgumentException(
+					String.format("Cannot convert from %s to %s.", this, other));
+		
+	}
+	
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Converts by multiplying by {@code conversionFactor}
+	 */
+	@Override
+	protected double convertToBase(final double value) {
+		return value * this.getConversionFactor();
+	}
+	
+	/**
+	 * Converts an {@code UncertainDouble} to the base unit.
+	 * 
+	 * @since 2020-09-07
+	 */
+	UncertainDouble convertToBase(final UncertainDouble value) {
+		return value.timesExact(this.getConversionFactor());
+	}
+	
+	/**
+	 * Divides this unit by a scalar.
+	 * 
+	 * @param divisor scalar to divide by
+	 * @return quotient
+	 * @since 2018-12-23
+	 * @since v0.1.0
+	 */
+	public LinearUnit dividedBy(final double divisor) {
+		return valueOf(this.getBase(), this.getConversionFactor() / divisor);
+	}
+	
+	/**
+	 * Returns the quotient of this unit and another.
+	 * 
+	 * @param divisor unit to divide by
+	 * @return quotient of two units
+	 * @throws NullPointerException if {@code divisor} is null
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public LinearUnit dividedBy(final LinearUnit divisor) {
+		Objects.requireNonNull(divisor, "other must not be null");
+		
+		// divide the units
+		final ObjectProduct<BaseUnit> base = this.getBase()
+				.dividedBy(divisor.getBase());
+		return valueOf(base,
+				this.getConversionFactor() / divisor.getConversionFactor());
+	}
+	
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses the base and conversion factor of units to test for equality.
+	 */
+	@Override
+	public boolean equals(final Object obj) {
+		if (!(obj instanceof LinearUnit))
+			return false;
+		final LinearUnit other = (LinearUnit) obj;
+		return Objects.equals(this.getBase(), other.getBase())
+				&& DecimalComparison.equals(this.getConversionFactor(),
+						other.getConversionFactor());
+	}
+	
+	/**
+	 * @return conversion factor
+	 * @since 2019-10-16
+	 */
+	public double getConversionFactor() {
+		return this.conversionFactor;
+	}
+	
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses the base and conversion factor to compute a hash code.
+	 */
+	@Override
+	public int hashCode() {
+		return 31 * this.getBase().hashCode()
+				+ DecimalComparison.hash(this.getConversionFactor());
+	}
+	
+	/**
+	 * @return whether this unit is equivalent to a {@code BaseUnit} (i.e. there
+	 *         is a {@code BaseUnit b} where
+	 *         {@code b.asLinearUnit().equals(this)} returns {@code true}.)
+	 * @since 2019-10-16
+	 */
+	public boolean isBase() {
+		return this.isCoherent() && this.getBase().isSingleObject();
+	}
+	
+	/**
+	 * @return whether this unit is coherent (i.e. has conversion factor 1)
+	 * @since 2019-10-16
+	 */
+	public boolean isCoherent() {
+		return this.getConversionFactor() == 1;
+	}
+	
+	/**
+	 * Returns the difference of this unit and another.
+	 * <p>
+	 * Two units can be subtracted if they have the same base. Note that
+	 * {@link #canConvertTo} can be used to determine this. If {@code subtrahend}
+	 * does not meet this condition, an {@code IllegalArgumentException} will be
+	 * thrown.
+	 * </p>
+	 * 
+	 * @param subtrahend unit to subtract
+	 * @return difference of units
+	 * @throws IllegalArgumentException if {@code subtrahend} is not compatible
+	 *                                  for subtraction as described above
+	 * @throws NullPointerException     if {@code subtrahend} is null
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	public LinearUnit minus(final LinearUnit subtrahend) {
+		Objects.requireNonNull(subtrahend, "addend must not be null.");
+		
+		// reject subtrahends that cannot be added to this unit
+		if (!this.getBase().equals(subtrahend.getBase()))
+			throw new IllegalArgumentException(String.format(
+					"Incompatible units for subtraction \"%s\" and \"%s\".", this,
+					subtrahend));
+		
+		// subtract the units
+		return valueOf(this.getBase(),
+				this.getConversionFactor() - subtrahend.getConversionFactor());
+	}
+	
+	/**
+	 * Returns the sum of this unit and another.
+	 * <p>
+	 * Two units can be added if they have the same base. Note that
+	 * {@link #canConvertTo} can be used to determine this. If {@code addend}
+	 * does not meet this condition, an {@code IllegalArgumentException} will be
+	 * thrown.
+	 * </p>
+	 * 
+	 * @param addend unit to add
+	 * @return sum of units
+	 * @throws IllegalArgumentException if {@code addend} is not compatible for
+	 *                                  addition as described above
+	 * @throws NullPointerException     if {@code addend} is null
+	 * @since 2019-03-17
+	 * @since v0.2.0
+	 */
+	public LinearUnit plus(final LinearUnit addend) {
+		Objects.requireNonNull(addend, "addend must not be null.");
+		
+		// reject addends that cannot be added to this unit
+		if (!this.getBase().equals(addend.getBase()))
+			throw new IllegalArgumentException(String.format(
+					"Incompatible units for addition \"%s\" and \"%s\".", this,
+					addend));
+		
+		// add the units
+		return valueOf(this.getBase(),
+				this.getConversionFactor() + addend.getConversionFactor());
+	}
+	
+	/**
+	 * Multiplies this unit by a scalar.
+	 * 
+	 * @param multiplier scalar to multiply by
+	 * @return product
+	 * @since 2018-12-23
+	 * @since v0.1.0
+	 */
+	public LinearUnit times(final double multiplier) {
+		return valueOf(this.getBase(), this.getConversionFactor() * multiplier);
+	}
+	
+	/**
+	 * Returns the product of this unit and another.
+	 * 
+	 * @param multiplier unit to multiply by
+	 * @return product of two units
+	 * @throws NullPointerException if {@code multiplier} is null
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public LinearUnit times(final LinearUnit multiplier) {
+		Objects.requireNonNull(multiplier, "other must not be null");
+		
+		// multiply the units
+		final ObjectProduct<BaseUnit> base = this.getBase()
+				.times(multiplier.getBase());
+		return valueOf(base,
+				this.getConversionFactor() * multiplier.getConversionFactor());
+	}
+	
+	/**
+	 * Returns this unit but to an exponent.
+	 * 
+	 * @param exponent exponent to exponentiate unit to
+	 * @return exponentiated unit
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	public LinearUnit toExponent(final int exponent) {
+		return valueOf(this.getBase().toExponent(exponent),
+				Math.pow(this.conversionFactor, exponent));
+	}
+	
+	/**
+	 * @return a string providing a definition of this unit
+	 * @since 2019-10-21
+	 */
+	@Override
+	public String toString() {
+		return this.getPrimaryName().orElse("Unnamed unit")
+				+ (this.getSymbol().isPresent()
+						? String.format(" (%s)", this.getSymbol().get())
+						: "")
+				+ ", " + Double.toString(this.conversionFactor) + " * "
+				+ this.getBase().toString(u -> u.getSymbol().get());
+	}
+	
+	@Override
+	public LinearUnit withName(final NameSymbol ns) {
+		return valueOf(this.getBase(), this.getConversionFactor(), ns);
+	}
+	
+	/**
+	 * Returns the result of applying {@code prefix} to this unit.
+	 * <p>
+	 * If this unit and the provided prefix have a primary name, the returned
+	 * unit will have a primary name (prefix's name + unit's name). <br>
+	 * If this unit and the provided prefix have a symbol, the returned unit will
+	 * have a symbol. <br>
+	 * This method ignores alternate names of both this unit and the provided
+	 * prefix.
+	 * 
+	 * @param prefix prefix to apply
+	 * @return unit with prefix
+	 * @since 2019-03-18
+	 * @since v0.2.0
+	 * @throws NullPointerException if prefix is null
+	 */
+	public LinearUnit withPrefix(final UnitPrefix prefix) {
+		final LinearUnit unit = this.times(prefix.getMultiplier());
+		
+		// create new name and symbol, if possible
+		final String name;
+		if (this.getPrimaryName().isPresent()
+				&& prefix.getPrimaryName().isPresent()) {
+			name = prefix.getPrimaryName().get() + this.getPrimaryName().get();
+		} else {
+			name = null;
+		}
+		
+		final String symbol;
+		if (this.getSymbol().isPresent() && prefix.getSymbol().isPresent()) {
+			symbol = prefix.getSymbol().get() + this.getSymbol().get();
+		} else {
+			symbol = null;
+		}
+		
+		return unit.withName(NameSymbol.ofNullable(name, symbol));
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/LinearUnitValue.java b/src/main/java/org/unitConverter/unit/LinearUnitValue.java
new file mode 100644
index 0000000..8de734e
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/LinearUnitValue.java
@@ -0,0 +1,341 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+import java.util.Optional;
+
+import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.UncertainDouble;
+
+/**
+ * A possibly uncertain value expressed in a linear unit.
+ * 
+ * Unless otherwise indicated, all methods in this class throw a
+ * {@code NullPointerException} when an argument is null.
+ * 
+ * @author Adrien Hopkins
+ * @since 2020-07-26
+ */
+public final class LinearUnitValue {
+	public static final LinearUnitValue ONE = getExact(SI.ONE, 1);
+	
+	/**
+	 * Gets an exact {@code LinearUnitValue}
+	 * 
+	 * @param unit  unit to express with
+	 * @param value value to express
+	 * @return exact {@code LinearUnitValue} instance
+	 * @since 2020-07-26
+	 */
+	public static final LinearUnitValue getExact(final LinearUnit unit,
+			final double value) {
+		return new LinearUnitValue(
+				Objects.requireNonNull(unit, "unit must not be null"),
+				UncertainDouble.of(value, 0));
+	}
+	
+	/**
+	 * Gets an uncertain {@code LinearUnitValue}
+	 * 
+	 * @param unit        unit to express with
+	 * @param value       value to express
+	 * @param uncertainty absolute uncertainty of value
+	 * @return uncertain {@code LinearUnitValue} instance
+	 * @since 2020-07-26
+	 */
+	public static final LinearUnitValue of(final LinearUnit unit,
+			final UncertainDouble value) {
+		return new LinearUnitValue(
+				Objects.requireNonNull(unit, "unit must not be null"),
+				Objects.requireNonNull(value, "value may not be null"));
+	}
+	
+	private final LinearUnit unit;
+	
+	private final UncertainDouble value;
+	
+	/**
+	 * @param unit  unit to express as
+	 * @param value value to express
+	 * @since 2020-07-26
+	 */
+	private LinearUnitValue(final LinearUnit unit, final UncertainDouble value) {
+		this.unit = unit;
+		this.value = value;
+	}
+	
+	/**
+	 * @return this value as a {@code UnitValue}. All uncertainty information is
+	 *         removed from the returned value.
+	 * @since 2020-08-04
+	 */
+	public final UnitValue asUnitValue() {
+		return UnitValue.of(this.unit, this.value.value());
+	}
+	
+	/**
+	 * @param other a {@code LinearUnit}
+	 * @return true iff this value can be represented with {@code other}.
+	 * @since 2020-07-26
+	 */
+	public final boolean canConvertTo(final LinearUnit other) {
+		return this.unit.canConvertTo(other);
+	}
+	
+	/**
+	 * Returns a LinearUnitValue that represents the same value expressed in a
+	 * different unit
+	 * 
+	 * @param other new unit to express value in
+	 * @return value expressed in {@code other}
+	 * @since 2020-07-26
+	 */
+	public final LinearUnitValue convertTo(final LinearUnit other) {
+		return LinearUnitValue.of(other, this.unit.convertTo(other, this.value));
+	}
+	
+	/**
+	 * Divides this value by a scalar
+	 * 
+	 * @param divisor value to divide by
+	 * @return multiplied value
+	 * @since 2020-07-28
+	 */
+	public LinearUnitValue dividedBy(final double divisor) {
+		return LinearUnitValue.of(this.unit, this.value.dividedByExact(divisor));
+	}
+	
+	/**
+	 * Divides this value by another value
+	 * 
+	 * @param divisor value to multiply by
+	 * @return quotient
+	 * @since 2020-07-28
+	 */
+	public LinearUnitValue dividedBy(final LinearUnitValue divisor) {
+		return LinearUnitValue.of(this.unit.dividedBy(divisor.unit),
+				this.value.dividedBy(divisor.value));
+	}
+	
+	/**
+	 * Returns true if this and obj represent the same value, regardless of
+	 * whether or not they are expressed in the same unit. So (1000 m).equals(1
+	 * km) returns true.
+	 * 
+	 * @since 2020-07-26
+	 * @see #equals(Object, boolean)
+	 */
+	@Override
+	public boolean equals(final Object obj) {
+		if (!(obj instanceof LinearUnitValue))
+			return false;
+		final LinearUnitValue other = (LinearUnitValue) obj;
+		return Objects.equals(this.unit.getBase(), other.unit.getBase())
+				&& this.unit.convertToBase(this.value)
+						.equals(other.unit.convertToBase(other.value));
+	}
+	
+	/**
+	 * Returns true if this and obj represent the same value, regardless of
+	 * whether or not they are expressed in the same unit. So (1000 m).equals(1
+	 * km) returns true.
+	 * <p>
+	 * If avoidFPErrors is true, this method will attempt to avoid floating-point
+	 * errors, at the cost of not always being transitive.
+	 * 
+	 * @since 2020-07-28
+	 */
+	public boolean equals(final Object obj, final boolean avoidFPErrors) {
+		if (!avoidFPErrors)
+			return this.equals(obj);
+		if (!(obj instanceof LinearUnitValue))
+			return false;
+		final LinearUnitValue other = (LinearUnitValue) obj;
+		return Objects.equals(this.unit.getBase(), other.unit.getBase())
+				&& DecimalComparison.equals(this.unit.convertToBase(this.value),
+						other.unit.convertToBase(other.value));
+	}
+	
+	/**
+	 * @param other another {@code LinearUnitValue}
+	 * @return true iff this and other are within each other's uncertainty range
+	 * 
+	 * @since 2020-07-26
+	 */
+	public boolean equivalent(final LinearUnitValue other) {
+		if (other == null
+				|| !Objects.equals(this.unit.getBase(), other.unit.getBase()))
+			return false;
+		final LinearUnit base = LinearUnit.valueOf(this.unit.getBase(), 1);
+		final LinearUnitValue thisBase = this.convertTo(base);
+		final LinearUnitValue otherBase = other.convertTo(base);
+		
+		return thisBase.value.equivalent(otherBase.value);
+	}
+	
+	/**
+	 * @return the unit
+	 * @since 2020-09-29
+	 */
+	public final LinearUnit getUnit() {
+		return this.unit;
+	}
+	
+	/**
+	 * @return the value
+	 * @since 2020-09-29
+	 */
+	public final UncertainDouble getValue() {
+		return this.value;
+	}
+	
+	/**
+	 * @return the exact value
+	 * @since 2020-09-07
+	 */
+	public final double getValueExact() {
+		return this.value.value();
+	}
+	
+	@Override
+	public int hashCode() {
+		return Objects.hash(this.unit.getBase(),
+				this.unit.convertToBase(this.getValue()));
+	}
+	
+	/**
+	 * Returns the difference of this value and another, expressed in this
+	 * value's unit
+	 * 
+	 * @param subtrahend value to subtract
+	 * @return difference of values
+	 * @throws IllegalArgumentException if {@code subtrahend} has a unit that is
+	 *                                  not compatible for addition
+	 * @since 2020-07-26
+	 */
+	public LinearUnitValue minus(final LinearUnitValue subtrahend) {
+		Objects.requireNonNull(subtrahend, "subtrahend may not be null");
+		
+		if (!this.canConvertTo(subtrahend.unit))
+			throw new IllegalArgumentException(String.format(
+					"Incompatible units for subtraction \"%s\" and \"%s\".",
+					this.unit, subtrahend.unit));
+		
+		final LinearUnitValue otherConverted = subtrahend.convertTo(this.unit);
+		return LinearUnitValue.of(this.unit,
+				this.value.minus(otherConverted.value));
+	}
+	
+	/**
+	 * Returns the sum of this value and another, expressed in this value's unit
+	 * 
+	 * @param addend value to add
+	 * @return sum of values
+	 * @throws IllegalArgumentException if {@code addend} has a unit that is not
+	 *                                  compatible for addition
+	 * @since 2020-07-26
+	 */
+	public LinearUnitValue plus(final LinearUnitValue addend) {
+		Objects.requireNonNull(addend, "addend may not be null");
+		
+		if (!this.canConvertTo(addend.unit))
+			throw new IllegalArgumentException(String.format(
+					"Incompatible units for addition \"%s\" and \"%s\".", this.unit,
+					addend.unit));
+		
+		final LinearUnitValue otherConverted = addend.convertTo(this.unit);
+		return LinearUnitValue.of(this.unit,
+				this.value.plus(otherConverted.value));
+	}
+	
+	/**
+	 * Multiplies this value by a scalar
+	 * 
+	 * @param multiplier value to multiply by
+	 * @return multiplied value
+	 * @since 2020-07-28
+	 */
+	public LinearUnitValue times(final double multiplier) {
+		return LinearUnitValue.of(this.unit, this.value.timesExact(multiplier));
+	}
+	
+	/**
+	 * Multiplies this value by another value
+	 * 
+	 * @param multiplier value to multiply by
+	 * @return product
+	 * @since 2020-07-28
+	 */
+	public LinearUnitValue times(final LinearUnitValue multiplier) {
+		return LinearUnitValue.of(this.unit.times(multiplier.unit),
+				this.value.times(multiplier.value));
+	}
+	
+	/**
+	 * Raises a value to an exponent
+	 * 
+	 * @param exponent exponent to raise to
+	 * @return result of exponentiation
+	 * @since 2020-07-28
+	 */
+	public LinearUnitValue toExponent(final int exponent) {
+		return LinearUnitValue.of(this.unit.toExponent(exponent),
+				this.value.toExponentExact(exponent));
+	}
+	
+	@Override
+	public String toString() {
+		return this.toString(!this.value.isExact());
+	}
+	
+	/**
+	 * Returns a string representing the object. <br>
+	 * If the attached unit has a name or symbol, the string looks like "12 km".
+	 * Otherwise, it looks like "13 unnamed unit (= 2 m/s)".
+	 * <p>
+	 * If showUncertainty is true, strings like "35 ± 8" are shown instead of
+	 * single numbers.
+	 * <p>
+	 * Non-exact values are rounded intelligently based on their uncertainty.
+	 * 
+	 * @since 2020-07-26
+	 */
+	public String toString(final boolean showUncertainty) {
+		final Optional<String> primaryName = this.unit.getPrimaryName();
+		final Optional<String> symbol = this.unit.getSymbol();
+		final String chosenName = symbol.orElse(primaryName.orElse(null));
+		
+		final UncertainDouble baseValue = this.unit.convertToBase(this.value);
+		
+		// get rounded strings
+		// if showUncertainty is true, add brackets around the string
+		final String valueString = showUncertainty ? "("
+				: "" + this.value.toString(showUncertainty)
+						+ (showUncertainty ? ")" : "");
+		final String baseValueString = showUncertainty ? "("
+				: "" + baseValue.toString(showUncertainty)
+						+ (showUncertainty ? ")" : "");
+		
+		// create string
+		if (primaryName.isEmpty() && symbol.isEmpty())
+			return String.format("%s unnamed unit (= %s %s)", valueString,
+					baseValueString, this.unit.getBase());
+		else
+			return String.format("%s %s", valueString, chosenName);
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/MultiUnit.java b/src/main/java/org/unitConverter/unit/MultiUnit.java
new file mode 100644
index 0000000..a1623f8
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/MultiUnit.java
@@ -0,0 +1,160 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.List;
+
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * A combination of units, like "5 foot + 7 inch". All but the last units should
+ * have a whole number value associated with them.
+ *
+ * @since 2020-10-02
+ */
+public final class MultiUnit extends Unitlike<List<Double>> {
+	/**
+	 * Creates a {@code MultiUnit} from its units. It will not have a name or
+	 * symbol.
+	 *
+	 * @since 2020-10-03
+	 */
+	public static final MultiUnit of(LinearUnit... units) {
+		return of(Arrays.asList(units));
+	}
+	
+	/**
+	 * Creates a {@code MultiUnit} from its units. It will not have a name or
+	 * symbol.
+	 *
+	 * @since 2020-10-03
+	 */
+	public static final MultiUnit of(List<LinearUnit> units) {
+		if (units.size() < 1)
+			throw new IllegalArgumentException("Must have at least one unit");
+		final ObjectProduct<BaseUnit> unitBase = units.get(0).getBase();
+		for (final LinearUnit unit : units) {
+			if (!unitBase.equals(unit.getBase()))
+				throw new IllegalArgumentException(
+						"All units must have the same base.");
+		}
+		return new MultiUnit(new ArrayList<>(units), unitBase, NameSymbol.EMPTY);
+	}
+	
+	/**
+	 * The units that make up this value.
+	 */
+	private final List<LinearUnit> units;
+	
+	/**
+	 * Creates a {@code MultiUnit}.
+	 *
+	 * @since 2020-10-03
+	 */
+	private MultiUnit(List<LinearUnit> units, ObjectProduct<BaseUnit> unitBase,
+			NameSymbol ns) {
+		super(unitBase, ns);
+		this.units = units;
+	}
+	
+	@Override
+	protected List<Double> convertFromBase(double value) {
+		final List<Double> values = new ArrayList<>(this.units.size());
+		double temp = value;
+		
+		for (final LinearUnit unit : this.units.subList(0,
+				this.units.size() - 1)) {
+			values.add(Math.floor(temp / unit.getConversionFactor()));
+			temp %= unit.getConversionFactor();
+		}
+		
+		values.add(this.units.size() - 1,
+				this.units.get(this.units.size() - 1).convertFromBase(temp));
+		
+		return values;
+	}
+	
+	/**
+	 * Converts a value expressed in this unitlike form to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 				
+	 * @param other unit to convert to
+	 * @param value value to convert
+	 * @return converted value
+	 * @since 2020-10-03
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unitlike form (as
+	 *                                  tested by {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final <U extends Unitlike<V>, V> V convertTo(U other,
+			double... values) {
+		final List<Double> valueList = new ArrayList<>(values.length);
+		for (final double d : values) {
+			valueList.add(d);
+		}
+		
+		return this.convertTo(other, valueList);
+	}
+	
+	/**
+	 * Converts a value expressed in this unitlike form to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 				
+	 * @param other unit to convert to
+	 * @param value value to convert
+	 * @return converted value
+	 * @since 2020-10-03
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unitlike form (as
+	 *                                  tested by {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final double convertTo(Unit other, double... values) {
+		final List<Double> valueList = new ArrayList<>(values.length);
+		for (final double d : values) {
+			valueList.add(d);
+		}
+		
+		return this.convertTo(other, valueList);
+	}
+	
+	@Override
+	protected double convertToBase(List<Double> value) {
+		if (value.size() != this.units.size())
+			throw new IllegalArgumentException("Wrong number of values for "
+					+ this.units.size() + "-unit MultiUnit.");
+		
+		double baseValue = 0;
+		for (int i = 0; i < this.units.size(); i++) {
+			baseValue += value.get(i) * this.units.get(i).getConversionFactor();
+		}
+		return baseValue;
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/NameSymbol.java b/src/main/java/org/unitConverter/unit/NameSymbol.java
new file mode 100644
index 0000000..8d8302a
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/NameSymbol.java
@@ -0,0 +1,280 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Arrays;
+import java.util.Collections;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * A class that can be used to specify names and a symbol for a unit.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-21
+ */
+public final class NameSymbol {
+	public static final NameSymbol EMPTY = new NameSymbol(Optional.empty(),
+			Optional.empty(), new HashSet<>());
+	
+	/**
+	 * Creates a {@code NameSymbol}, ensuring that if primaryName is null and
+	 * otherNames is not empty, one name is moved from otherNames to primaryName
+	 * 
+	 * Ensure that otherNames is a copy of the inputted argument.
+	 */
+	private static final NameSymbol create(final String name,
+			final String symbol, final Set<String> otherNames) {
+		final Optional<String> primaryName;
+		
+		if (name == null && !otherNames.isEmpty()) {
+			// get primary name and remove it from savedNames
+			final Iterator<String> it = otherNames.iterator();
+			assert it.hasNext();
+			primaryName = Optional.of(it.next());
+			otherNames.remove(primaryName.get());
+		} else {
+			primaryName = Optional.ofNullable(name);
+		}
+		
+		return new NameSymbol(primaryName, Optional.ofNullable(symbol),
+				otherNames);
+	}
+	
+	/**
+	 * Gets a {@code NameSymbol} with a primary name, a symbol and no other
+	 * names.
+	 * 
+	 * @param name   name to use
+	 * @param symbol symbol to use
+	 * @return NameSymbol instance
+	 * @since 2019-10-21
+	 * @throws NullPointerException if name or symbol is null
+	 */
+	public static final NameSymbol of(final String name, final String symbol) {
+		return new NameSymbol(Optional.of(name), Optional.of(symbol),
+				new HashSet<>());
+	}
+	
+	/**
+	 * Gets a {@code NameSymbol} with a primary name, a symbol and additional
+	 * names.
+	 * 
+	 * @param name       name to use
+	 * @param symbol     symbol to use
+	 * @param otherNames other names to use
+	 * @return NameSymbol instance
+	 * @since 2019-10-21
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final NameSymbol of(final String name, final String symbol,
+			final Set<String> otherNames) {
+		return new NameSymbol(Optional.of(name), Optional.of(symbol),
+				new HashSet<>(Objects.requireNonNull(otherNames,
+						"otherNames must not be null.")));
+	}
+	
+	/**
+	 * h * Gets a {@code NameSymbol} with a primary name, a symbol and additional
+	 * names.
+	 * 
+	 * @param name       name to use
+	 * @param symbol     symbol to use
+	 * @param otherNames other names to use
+	 * @return NameSymbol instance
+	 * @since 2019-10-21
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final NameSymbol of(final String name, final String symbol,
+			final String... otherNames) {
+		return new NameSymbol(Optional.of(name), Optional.of(symbol),
+				new HashSet<>(Arrays.asList(Objects.requireNonNull(otherNames,
+						"otherNames must not be null."))));
+	}
+	
+	/**
+	 * Gets a {@code NameSymbol} with a primary name, no symbol, and no other
+	 * names.
+	 * 
+	 * @param name name to use
+	 * @return NameSymbol instance
+	 * @since 2019-10-21
+	 * @throws NullPointerException if name is null
+	 */
+	public static final NameSymbol ofName(final String name) {
+		return new NameSymbol(Optional.of(name), Optional.empty(),
+				new HashSet<>());
+	}
+	
+	/**
+	 * Gets a {@code NameSymbol} with a primary name, a symbol and additional
+	 * names.
+	 * <p>
+	 * If any argument is null, this static factory replaces it with an empty
+	 * Optional or empty Set.
+	 * <p>
+	 * If {@code name} is null and {@code otherNames} is not empty, a primary
+	 * name will be picked from {@code otherNames}. This name will not appear in
+	 * getOtherNames().
+	 * 
+	 * @param name       name to use
+	 * @param symbol     symbol to use
+	 * @param otherNames other names to use
+	 * @return NameSymbol instance
+	 * @since 2019-11-26
+	 */
+	public static final NameSymbol ofNullable(final String name,
+			final String symbol, final Set<String> otherNames) {
+		return NameSymbol.create(name, symbol,
+				otherNames == null ? new HashSet<>() : new HashSet<>(otherNames));
+	}
+	
+	/**
+	 * h * Gets a {@code NameSymbol} with a primary name, a symbol and additional
+	 * names.
+	 * <p>
+	 * If any argument is null, this static factory replaces it with an empty
+	 * Optional or empty Set.
+	 * <p>
+	 * If {@code name} is null and {@code otherNames} is not empty, a primary
+	 * name will be picked from {@code otherNames}. This name will not appear in
+	 * getOtherNames().
+	 * 
+	 * @param name       name to use
+	 * @param symbol     symbol to use
+	 * @param otherNames other names to use
+	 * @return NameSymbol instance
+	 * @since 2019-11-26
+	 */
+	public static final NameSymbol ofNullable(final String name,
+			final String symbol, final String... otherNames) {
+		return create(name, symbol, otherNames == null ? new HashSet<>()
+				: new HashSet<>(Arrays.asList(otherNames)));
+	}
+	
+	/**
+	 * Gets a {@code NameSymbol} with a symbol and no names.
+	 * 
+	 * @param symbol symbol to use
+	 * @return NameSymbol instance
+	 * @since 2019-10-21
+	 * @throws NullPointerException if symbol is null
+	 */
+	public static final NameSymbol ofSymbol(final String symbol) {
+		return new NameSymbol(Optional.empty(), Optional.of(symbol),
+				new HashSet<>());
+	}
+	
+	private final Optional<String> primaryName;
+	private final Optional<String> symbol;
+	
+	private final Set<String> otherNames;
+	
+	/**
+	 * Creates the {@code NameSymbol}.
+	 * 
+	 * @param primaryName primary name of unit
+	 * @param symbol      symbol used to represent unit
+	 * @param otherNames  other names and/or spellings, should be a mutable copy
+	 *                    of the argument
+	 * @since 2019-10-21
+	 */
+	private NameSymbol(final Optional<String> primaryName,
+			final Optional<String> symbol, final Set<String> otherNames) {
+		this.primaryName = primaryName;
+		this.symbol = symbol;
+		otherNames.remove(null);
+		this.otherNames = Collections.unmodifiableSet(otherNames);
+		
+		if (this.primaryName.isEmpty()) {
+			assert this.otherNames.isEmpty();
+		}
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (!(obj instanceof NameSymbol))
+			return false;
+		final NameSymbol other = (NameSymbol) obj;
+		if (this.otherNames == null) {
+			if (other.otherNames != null)
+				return false;
+		} else if (!this.otherNames.equals(other.otherNames))
+			return false;
+		if (this.primaryName == null) {
+			if (other.primaryName != null)
+				return false;
+		} else if (!this.primaryName.equals(other.primaryName))
+			return false;
+		if (this.symbol == null) {
+			if (other.symbol != null)
+				return false;
+		} else if (!this.symbol.equals(other.symbol))
+			return false;
+		return true;
+	}
+	
+	/**
+	 * @return otherNames
+	 * @since 2019-10-21
+	 */
+	public final Set<String> getOtherNames() {
+		return this.otherNames;
+	}
+	
+	/**
+	 * @return primaryName
+	 * @since 2019-10-21
+	 */
+	public final Optional<String> getPrimaryName() {
+		return this.primaryName;
+	}
+	
+	/**
+	 * @return symbol
+	 * @since 2019-10-21
+	 */
+	public final Optional<String> getSymbol() {
+		return this.symbol;
+	}
+	
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result
+				+ (this.otherNames == null ? 0 : this.otherNames.hashCode());
+		result = prime * result
+				+ (this.primaryName == null ? 0 : this.primaryName.hashCode());
+		result = prime * result
+				+ (this.symbol == null ? 0 : this.symbol.hashCode());
+		return result;
+	}
+	
+	/**
+	 * @return true iff this {@code NameSymbol} contains no names or symbols.
+	 */
+	public final boolean isEmpty() {
+		// if primaryName is empty, otherNames must also be empty
+		return this.primaryName.isEmpty() && this.symbol.isEmpty();
+	}
+}
\ No newline at end of file
diff --git a/src/main/java/org/unitConverter/unit/Nameable.java b/src/main/java/org/unitConverter/unit/Nameable.java
new file mode 100644
index 0000000..36740ab
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/Nameable.java
@@ -0,0 +1,59 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Optional;
+import java.util.Set;
+
+/**
+ * An object that can hold one or more names, and possibly a symbol. The name
+ * and symbol data should be immutable.
+ *
+ * @since 2020-09-07
+ */
+public interface Nameable {
+	/**
+	 * @return a {@code NameSymbol} that contains this object's primary name,
+	 *         symbol and other names
+	 * @since 2020-09-07
+	 */
+	NameSymbol getNameSymbol();
+	
+	/**
+	 * @return set of alternate names
+	 * @since 2020-09-07
+	 */
+	default Set<String> getOtherNames() {
+		return this.getNameSymbol().getOtherNames();
+	}
+	
+	/**
+	 * @return preferred name of object
+	 * @since 2020-09-07
+	 */
+	default Optional<String> getPrimaryName() {
+		return this.getNameSymbol().getPrimaryName();
+	}
+	
+	/**
+	 * @return short symbol representing object
+	 * @since 2020-09-07
+	 */
+	default Optional<String> getSymbol() {
+		return this.getNameSymbol().getSymbol();
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/SI.java b/src/main/java/org/unitConverter/unit/SI.java
new file mode 100644
index 0000000..81736f3
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/SI.java
@@ -0,0 +1,479 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Set;
+
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * All of the units, prefixes and dimensions that are used by the SI, as well as
+ * some outside the SI.
+ * 
+ * <p>
+ * This class does not include prefixed units. To obtain prefixed units, use
+ * {@link LinearUnit#withPrefix}:
+ * 
+ * <pre>
+ * LinearUnit KILOMETRE = SI.METRE.withPrefix(SI.KILO);
+ * </pre>
+ * 
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class SI {
+	/// dimensions used by SI units
+	// base dimensions, as BaseDimensions
+	public static final class BaseDimensions {
+		public static final BaseDimension LENGTH = BaseDimension.valueOf("Length",
+				"L");
+		public static final BaseDimension MASS = BaseDimension.valueOf("Mass",
+				"M");
+		public static final BaseDimension TIME = BaseDimension.valueOf("Time",
+				"T");
+		public static final BaseDimension ELECTRIC_CURRENT = BaseDimension
+				.valueOf("Electric Current", "I");
+		public static final BaseDimension TEMPERATURE = BaseDimension
+				.valueOf("Temperature", "\u0398"); // theta symbol
+		public static final BaseDimension QUANTITY = BaseDimension
+				.valueOf("Quantity", "N");
+		public static final BaseDimension LUMINOUS_INTENSITY = BaseDimension
+				.valueOf("Luminous Intensity", "J");
+		public static final BaseDimension INFORMATION = BaseDimension
+				.valueOf("Information", "Info"); // non-SI
+		public static final BaseDimension CURRENCY = BaseDimension
+				.valueOf("Currency", "$$"); // non-SI
+		
+		// You may NOT get SI.BaseDimensions instances!
+		private BaseDimensions() {
+			throw new AssertionError();
+		}
+	}
+	
+	/// base units of the SI
+	// suppressing warnings since these are the same object, but in a different
+	/// form (class)
+	@SuppressWarnings("hiding")
+	public static final class BaseUnits {
+		public static final BaseUnit METRE = BaseUnit
+				.valueOf(BaseDimensions.LENGTH, "metre", "m");
+		public static final BaseUnit KILOGRAM = BaseUnit
+				.valueOf(BaseDimensions.MASS, "kilogram", "kg");
+		public static final BaseUnit SECOND = BaseUnit
+				.valueOf(BaseDimensions.TIME, "second", "s");
+		public static final BaseUnit AMPERE = BaseUnit
+				.valueOf(BaseDimensions.ELECTRIC_CURRENT, "ampere", "A");
+		public static final BaseUnit KELVIN = BaseUnit
+				.valueOf(BaseDimensions.TEMPERATURE, "kelvin", "K");
+		public static final BaseUnit MOLE = BaseUnit
+				.valueOf(BaseDimensions.QUANTITY, "mole", "mol");
+		public static final BaseUnit CANDELA = BaseUnit
+				.valueOf(BaseDimensions.LUMINOUS_INTENSITY, "candela", "cd");
+		public static final BaseUnit BIT = BaseUnit
+				.valueOf(BaseDimensions.INFORMATION, "bit", "b");
+		public static final BaseUnit DOLLAR = BaseUnit
+				.valueOf(BaseDimensions.CURRENCY, "dollar", "$");
+		
+		public static final Set<BaseUnit> BASE_UNITS = Set.of(METRE, KILOGRAM,
+				SECOND, AMPERE, KELVIN, MOLE, CANDELA, BIT);
+		
+		// You may NOT get SI.BaseUnits instances!
+		private BaseUnits() {
+			throw new AssertionError();
+		}
+	}
+	
+	/**
+	 * Constants that relate to the SI or other systems.
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Constants {
+		public static final LinearUnit EARTH_GRAVITY = METRE.dividedBy(SECOND)
+				.dividedBy(SECOND).times(9.80665);
+	}
+	
+	// dimensions used in the SI, as ObjectProducts
+	public static final class Dimensions {
+		public static final ObjectProduct<BaseDimension> EMPTY = ObjectProduct
+				.empty();
+		public static final ObjectProduct<BaseDimension> LENGTH = ObjectProduct
+				.oneOf(BaseDimensions.LENGTH);
+		public static final ObjectProduct<BaseDimension> MASS = ObjectProduct
+				.oneOf(BaseDimensions.MASS);
+		public static final ObjectProduct<BaseDimension> TIME = ObjectProduct
+				.oneOf(BaseDimensions.TIME);
+		public static final ObjectProduct<BaseDimension> ELECTRIC_CURRENT = ObjectProduct
+				.oneOf(BaseDimensions.ELECTRIC_CURRENT);
+		public static final ObjectProduct<BaseDimension> TEMPERATURE = ObjectProduct
+				.oneOf(BaseDimensions.TEMPERATURE);
+		public static final ObjectProduct<BaseDimension> QUANTITY = ObjectProduct
+				.oneOf(BaseDimensions.QUANTITY);
+		public static final ObjectProduct<BaseDimension> LUMINOUS_INTENSITY = ObjectProduct
+				.oneOf(BaseDimensions.LUMINOUS_INTENSITY);
+		public static final ObjectProduct<BaseDimension> INFORMATION = ObjectProduct
+				.oneOf(BaseDimensions.INFORMATION);
+		public static final ObjectProduct<BaseDimension> CURRENCY = ObjectProduct
+				.oneOf(BaseDimensions.CURRENCY);
+		
+		// derived dimensions without named SI units
+		public static final ObjectProduct<BaseDimension> AREA = LENGTH
+				.times(LENGTH);
+		public static final ObjectProduct<BaseDimension> VOLUME = AREA
+				.times(LENGTH);
+		public static final ObjectProduct<BaseDimension> VELOCITY = LENGTH
+				.dividedBy(TIME);
+		public static final ObjectProduct<BaseDimension> ACCELERATION = VELOCITY
+				.dividedBy(TIME);
+		public static final ObjectProduct<BaseDimension> WAVENUMBER = EMPTY
+				.dividedBy(LENGTH);
+		public static final ObjectProduct<BaseDimension> MASS_DENSITY = MASS
+				.dividedBy(VOLUME);
+		public static final ObjectProduct<BaseDimension> SURFACE_DENSITY = MASS
+				.dividedBy(AREA);
+		public static final ObjectProduct<BaseDimension> SPECIFIC_VOLUME = VOLUME
+				.dividedBy(MASS);
+		public static final ObjectProduct<BaseDimension> CURRENT_DENSITY = ELECTRIC_CURRENT
+				.dividedBy(AREA);
+		public static final ObjectProduct<BaseDimension> MAGNETIC_FIELD_STRENGTH = ELECTRIC_CURRENT
+				.dividedBy(LENGTH);
+		public static final ObjectProduct<BaseDimension> CONCENTRATION = QUANTITY
+				.dividedBy(VOLUME);
+		public static final ObjectProduct<BaseDimension> MASS_CONCENTRATION = CONCENTRATION
+				.times(MASS);
+		public static final ObjectProduct<BaseDimension> LUMINANCE = LUMINOUS_INTENSITY
+				.dividedBy(AREA);
+		public static final ObjectProduct<BaseDimension> REFRACTIVE_INDEX = VELOCITY
+				.dividedBy(VELOCITY);
+		public static final ObjectProduct<BaseDimension> REFRACTIVE_PERMEABILITY = EMPTY
+				.times(EMPTY);
+		public static final ObjectProduct<BaseDimension> ANGLE = LENGTH
+				.dividedBy(LENGTH);
+		public static final ObjectProduct<BaseDimension> SOLID_ANGLE = AREA
+				.dividedBy(AREA);
+		
+		// derived dimensions with named SI units
+		public static final ObjectProduct<BaseDimension> FREQUENCY = EMPTY
+				.dividedBy(TIME);
+		public static final ObjectProduct<BaseDimension> FORCE = MASS
+				.times(ACCELERATION);
+		public static final ObjectProduct<BaseDimension> ENERGY = FORCE
+				.times(LENGTH);
+		public static final ObjectProduct<BaseDimension> POWER = ENERGY
+				.dividedBy(TIME);
+		public static final ObjectProduct<BaseDimension> ELECTRIC_CHARGE = ELECTRIC_CURRENT
+				.times(TIME);
+		public static final ObjectProduct<BaseDimension> VOLTAGE = ENERGY
+				.dividedBy(ELECTRIC_CHARGE);
+		public static final ObjectProduct<BaseDimension> CAPACITANCE = ELECTRIC_CHARGE
+				.dividedBy(VOLTAGE);
+		public static final ObjectProduct<BaseDimension> ELECTRIC_RESISTANCE = VOLTAGE
+				.dividedBy(ELECTRIC_CURRENT);
+		public static final ObjectProduct<BaseDimension> ELECTRIC_CONDUCTANCE = ELECTRIC_CURRENT
+				.dividedBy(VOLTAGE);
+		public static final ObjectProduct<BaseDimension> MAGNETIC_FLUX = VOLTAGE
+				.times(TIME);
+		public static final ObjectProduct<BaseDimension> MAGNETIC_FLUX_DENSITY = MAGNETIC_FLUX
+				.dividedBy(AREA);
+		public static final ObjectProduct<BaseDimension> INDUCTANCE = MAGNETIC_FLUX
+				.dividedBy(ELECTRIC_CURRENT);
+		public static final ObjectProduct<BaseDimension> LUMINOUS_FLUX = LUMINOUS_INTENSITY
+				.times(SOLID_ANGLE);
+		public static final ObjectProduct<BaseDimension> ILLUMINANCE = LUMINOUS_FLUX
+				.dividedBy(AREA);
+		public static final ObjectProduct<BaseDimension> SPECIFIC_ENERGY = ENERGY
+				.dividedBy(MASS);
+		public static final ObjectProduct<BaseDimension> CATALYTIC_ACTIVITY = QUANTITY
+				.dividedBy(TIME);
+		
+		// You may NOT get SI.Dimension instances!
+		private Dimensions() {
+			throw new AssertionError();
+		}
+	}
+	
+	/// The units of the SI
+	public static final LinearUnit ONE = LinearUnit
+			.valueOf(ObjectProduct.empty(), 1);
+	
+	public static final LinearUnit METRE = BaseUnits.METRE.asLinearUnit()
+			.withName(NameSymbol.of("metre", "m", "meter"));
+	public static final LinearUnit KILOGRAM = BaseUnits.KILOGRAM.asLinearUnit()
+			.withName(NameSymbol.of("kilogram", "kg"));
+	public static final LinearUnit SECOND = BaseUnits.SECOND.asLinearUnit()
+			.withName(NameSymbol.of("second", "s", "sec"));
+	public static final LinearUnit AMPERE = BaseUnits.AMPERE.asLinearUnit()
+			.withName(NameSymbol.of("ampere", "A"));
+	public static final LinearUnit KELVIN = BaseUnits.KELVIN.asLinearUnit()
+			.withName(NameSymbol.of("kelvin", "K"));
+	public static final LinearUnit MOLE = BaseUnits.MOLE.asLinearUnit()
+			.withName(NameSymbol.of("mole", "mol"));
+	public static final LinearUnit CANDELA = BaseUnits.CANDELA.asLinearUnit()
+			.withName(NameSymbol.of("candela", "cd"));
+	public static final LinearUnit BIT = BaseUnits.BIT.asLinearUnit()
+			.withName(NameSymbol.of("bit", "b"));
+	public static final LinearUnit DOLLAR = BaseUnits.DOLLAR.asLinearUnit()
+			.withName(NameSymbol.of("dollar", "$"));
+	// Non-base units
+	public static final LinearUnit RADIAN = METRE.dividedBy(METRE)
+			.withName(NameSymbol.of("radian", "rad"));
+	
+	public static final LinearUnit STERADIAN = RADIAN.times(RADIAN)
+			.withName(NameSymbol.of("steradian", "sr"));
+	public static final LinearUnit HERTZ = ONE.dividedBy(SECOND)
+			.withName(NameSymbol.of("hertz", "Hz"));
+	// for periodic phenomena
+	public static final LinearUnit NEWTON = KILOGRAM.times(METRE)
+			.dividedBy(SECOND.times(SECOND))
+			.withName(NameSymbol.of("newton", "N"));
+	public static final LinearUnit PASCAL = NEWTON.dividedBy(METRE.times(METRE))
+			.withName(NameSymbol.of("pascal", "Pa"));
+	public static final LinearUnit JOULE = NEWTON.times(METRE)
+			.withName(NameSymbol.of("joule", "J"));
+	public static final LinearUnit WATT = JOULE.dividedBy(SECOND)
+			.withName(NameSymbol.of("watt", "W"));
+	public static final LinearUnit COULOMB = AMPERE.times(SECOND)
+			.withName(NameSymbol.of("coulomb", "C"));
+	public static final LinearUnit VOLT = JOULE.dividedBy(COULOMB)
+			.withName(NameSymbol.of("volt", "V"));
+	public static final LinearUnit FARAD = COULOMB.dividedBy(VOLT)
+			.withName(NameSymbol.of("farad", "F"));
+	public static final LinearUnit OHM = VOLT.dividedBy(AMPERE)
+			.withName(NameSymbol.of("ohm", "\u03A9")); // omega
+	public static final LinearUnit SIEMENS = ONE.dividedBy(OHM)
+			.withName(NameSymbol.of("siemens", "S"));
+	public static final LinearUnit WEBER = VOLT.times(SECOND)
+			.withName(NameSymbol.of("weber", "Wb"));
+	public static final LinearUnit TESLA = WEBER.dividedBy(METRE.times(METRE))
+			.withName(NameSymbol.of("tesla", "T"));
+	public static final LinearUnit HENRY = WEBER.dividedBy(AMPERE)
+			.withName(NameSymbol.of("henry", "H"));
+	public static final LinearUnit LUMEN = CANDELA.times(STERADIAN)
+			.withName(NameSymbol.of("lumen", "lm"));
+	public static final LinearUnit LUX = LUMEN.dividedBy(METRE.times(METRE))
+			.withName(NameSymbol.of("lux", "lx"));
+	public static final LinearUnit BEQUEREL = ONE.dividedBy(SECOND)
+			.withName(NameSymbol.of("bequerel", "Bq"));
+	// for activity referred to a nucleotide
+	public static final LinearUnit GRAY = JOULE.dividedBy(KILOGRAM)
+			.withName(NameSymbol.of("grey", "Gy"));
+	// for absorbed dose
+	public static final LinearUnit SIEVERT = JOULE.dividedBy(KILOGRAM)
+			.withName(NameSymbol.of("sievert", "Sv"));
+	// for dose equivalent
+	public static final LinearUnit KATAL = MOLE.dividedBy(SECOND)
+			.withName(NameSymbol.of("katal", "kat"));
+	// common derived units included for convenience
+	public static final LinearUnit GRAM = KILOGRAM.dividedBy(1000)
+			.withName(NameSymbol.of("gram", "g"));
+	
+	public static final LinearUnit SQUARE_METRE = METRE.toExponent(2)
+			.withName(NameSymbol.of("square metre", "m^2", "square meter",
+					"metre squared", "meter squared"));
+	public static final LinearUnit CUBIC_METRE = METRE.toExponent(3)
+			.withName(NameSymbol.of("cubic metre", "m^3", "cubic meter",
+					"metre cubed", "meter cubed"));
+	public static final LinearUnit METRE_PER_SECOND = METRE.dividedBy(SECOND)
+			.withName(
+					NameSymbol.of("metre per second", "m/s", "meter per second"));
+	// Non-SI units included for convenience
+	public static final Unit CELSIUS = Unit
+			.fromConversionFunctions(KELVIN.getBase(), tempK -> tempK - 273.15,
+					tempC -> tempC + 273.15)
+			.withName(NameSymbol.of("degree Celsius", "\u00B0C"));
+	
+	public static final LinearUnit MINUTE = SECOND.times(60)
+			.withName(NameSymbol.of("minute", "min"));
+	public static final LinearUnit HOUR = MINUTE.times(60)
+			.withName(NameSymbol.of("hour", "h", "hr"));
+	public static final LinearUnit DAY = HOUR.times(60)
+			.withName(NameSymbol.of("day", "d"));
+	public static final LinearUnit KILOMETRE_PER_HOUR = METRE.times(1000)
+			.dividedBy(HOUR).withName(NameSymbol.of("kilometre per hour", "km/h",
+					"kilometer per hour"));
+	public static final LinearUnit DEGREE = RADIAN.times(360 / (2 * Math.PI))
+			.withName(NameSymbol.of("degree", "\u00B0", "deg"));
+	public static final LinearUnit ARCMINUTE = DEGREE.dividedBy(60)
+			.withName(NameSymbol.of("arcminute", "arcmin"));
+	public static final LinearUnit ARCSECOND = ARCMINUTE.dividedBy(60)
+			.withName(NameSymbol.of("arcsecond", "arcsec"));
+	public static final LinearUnit ASTRONOMICAL_UNIT = METRE
+			.times(149597870700.0)
+			.withName(NameSymbol.of("astronomical unit", "au"));
+	public static final LinearUnit PARSEC = ASTRONOMICAL_UNIT
+			.dividedBy(ARCSECOND).withName(NameSymbol.of("parsec", "pc"));
+	public static final LinearUnit HECTARE = METRE.times(METRE).times(10000.0)
+			.withName(NameSymbol.of("hectare", "ha"));
+	public static final LinearUnit LITRE = METRE.times(METRE).times(METRE)
+			.dividedBy(1000.0).withName(NameSymbol.of("litre", "L", "l", "liter"));
+	public static final LinearUnit TONNE = KILOGRAM.times(1000.0)
+			.withName(NameSymbol.of("tonne", "t", "metric ton"));
+	public static final LinearUnit DALTON = KILOGRAM.times(1.660539040e-27)
+			.withName(NameSymbol.of("dalton", "Da", "atomic unit", "u")); // approximate
+	// value
+	public static final LinearUnit ELECTRONVOLT = JOULE.times(1.602176634e-19)
+			.withName(NameSymbol.of("electron volt", "eV"));
+	public static final LinearUnit BYTE = BIT.times(8)
+			.withName(NameSymbol.of("byte", "B"));
+	public static final Unit NEPER = Unit.fromConversionFunctions(ONE.getBase(),
+			pr -> 0.5 * Math.log(pr), Np -> Math.exp(2 * Np))
+			.withName(NameSymbol.of("neper", "Np"));
+	public static final Unit BEL = Unit.fromConversionFunctions(ONE.getBase(),
+			pr -> Math.log10(pr), dB -> Math.pow(10, dB))
+			.withName(NameSymbol.of("bel", "B"));
+	public static final Unit DECIBEL = Unit
+			.fromConversionFunctions(ONE.getBase(), pr -> 10 * Math.log10(pr),
+					dB -> Math.pow(10, dB / 10))
+			.withName(NameSymbol.of("decibel", "dB"));
+	
+	/// The prefixes of the SI
+	// expanding decimal prefixes
+	public static final UnitPrefix KILO = UnitPrefix.valueOf(1e3)
+			.withName(NameSymbol.of("kilo", "k", "K"));
+	public static final UnitPrefix MEGA = UnitPrefix.valueOf(1e6)
+			.withName(NameSymbol.of("mega", "M"));
+	public static final UnitPrefix GIGA = UnitPrefix.valueOf(1e9)
+			.withName(NameSymbol.of("giga", "G"));
+	public static final UnitPrefix TERA = UnitPrefix.valueOf(1e12)
+			.withName(NameSymbol.of("tera", "T"));
+	public static final UnitPrefix PETA = UnitPrefix.valueOf(1e15)
+			.withName(NameSymbol.of("peta", "P"));
+	public static final UnitPrefix EXA = UnitPrefix.valueOf(1e18)
+			.withName(NameSymbol.of("exa", "E"));
+	public static final UnitPrefix ZETTA = UnitPrefix.valueOf(1e21)
+			.withName(NameSymbol.of("zetta", "Z"));
+	public static final UnitPrefix YOTTA = UnitPrefix.valueOf(1e24)
+			.withName(NameSymbol.of("yotta", "Y"));
+	
+	// contracting decimal prefixes
+	public static final UnitPrefix MILLI = UnitPrefix.valueOf(1e-3)
+			.withName(NameSymbol.of("milli", "m"));
+	public static final UnitPrefix MICRO = UnitPrefix.valueOf(1e-6)
+			.withName(NameSymbol.of("micro", "\u03BC", "u")); // mu
+	public static final UnitPrefix NANO = UnitPrefix.valueOf(1e-9)
+			.withName(NameSymbol.of("nano", "n"));
+	public static final UnitPrefix PICO = UnitPrefix.valueOf(1e-12)
+			.withName(NameSymbol.of("pico", "p"));
+	public static final UnitPrefix FEMTO = UnitPrefix.valueOf(1e-15)
+			.withName(NameSymbol.of("femto", "f"));
+	public static final UnitPrefix ATTO = UnitPrefix.valueOf(1e-18)
+			.withName(NameSymbol.of("atto", "a"));
+	public static final UnitPrefix ZEPTO = UnitPrefix.valueOf(1e-21)
+			.withName(NameSymbol.of("zepto", "z"));
+	public static final UnitPrefix YOCTO = UnitPrefix.valueOf(1e-24)
+			.withName(NameSymbol.of("yocto", "y"));
+	
+	// prefixes that don't match the pattern of thousands
+	public static final UnitPrefix DEKA = UnitPrefix.valueOf(1e1)
+			.withName(NameSymbol.of("deka", "da", "deca", "D"));
+	public static final UnitPrefix HECTO = UnitPrefix.valueOf(1e2)
+			.withName(NameSymbol.of("hecto", "h", "H", "hekto"));
+	public static final UnitPrefix DECI = UnitPrefix.valueOf(1e-1)
+			.withName(NameSymbol.of("deci", "d"));
+	public static final UnitPrefix CENTI = UnitPrefix.valueOf(1e-2)
+			.withName(NameSymbol.of("centi", "c"));
+	public static final UnitPrefix KIBI = UnitPrefix.valueOf(1024)
+			.withName(NameSymbol.of("kibi", "Ki"));
+	public static final UnitPrefix MEBI = KIBI.times(1024)
+			.withName(NameSymbol.of("mebi", "Mi"));
+	public static final UnitPrefix GIBI = MEBI.times(1024)
+			.withName(NameSymbol.of("gibi", "Gi"));
+	public static final UnitPrefix TEBI = GIBI.times(1024)
+			.withName(NameSymbol.of("tebi", "Ti"));
+	public static final UnitPrefix PEBI = TEBI.times(1024)
+			.withName(NameSymbol.of("pebi", "Pi"));
+	public static final UnitPrefix EXBI = PEBI.times(1024)
+			.withName(NameSymbol.of("exbi", "Ei"));
+	
+	// a few prefixed units
+	public static final LinearUnit MICROMETRE = SI.METRE.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIMETRE = SI.METRE.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOMETRE = SI.METRE.withPrefix(SI.KILO);
+	public static final LinearUnit MEGAMETRE = SI.METRE.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROLITRE = SI.LITRE.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLILITRE = SI.LITRE.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOLITRE = SI.LITRE.withPrefix(SI.KILO);
+	public static final LinearUnit MEGALITRE = SI.LITRE.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROSECOND = SI.SECOND.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLISECOND = SI.SECOND.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOSECOND = SI.SECOND.withPrefix(SI.KILO);
+	public static final LinearUnit MEGASECOND = SI.SECOND.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROGRAM = SI.GRAM.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIGRAM = SI.GRAM.withPrefix(SI.MILLI);
+	public static final LinearUnit MEGAGRAM = SI.GRAM.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICRONEWTON = SI.NEWTON.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLINEWTON = SI.NEWTON.withPrefix(SI.MILLI);
+	public static final LinearUnit KILONEWTON = SI.NEWTON.withPrefix(SI.KILO);
+	public static final LinearUnit MEGANEWTON = SI.NEWTON.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROJOULE = SI.JOULE.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIJOULE = SI.JOULE.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOJOULE = SI.JOULE.withPrefix(SI.KILO);
+	public static final LinearUnit MEGAJOULE = SI.JOULE.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROWATT = SI.WATT.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIWATT = SI.WATT.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOWATT = SI.WATT.withPrefix(SI.KILO);
+	public static final LinearUnit MEGAWATT = SI.WATT.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROCOULOMB = SI.COULOMB
+			.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLICOULOMB = SI.COULOMB
+			.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOCOULOMB = SI.COULOMB.withPrefix(SI.KILO);
+	public static final LinearUnit MEGACOULOMB = SI.COULOMB.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit MICROAMPERE = SI.AMPERE.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIAMPERE = SI.AMPERE.withPrefix(SI.MILLI);
+	
+	public static final LinearUnit MICROVOLT = SI.VOLT.withPrefix(SI.MICRO);
+	public static final LinearUnit MILLIVOLT = SI.VOLT.withPrefix(SI.MILLI);
+	public static final LinearUnit KILOVOLT = SI.VOLT.withPrefix(SI.KILO);
+	public static final LinearUnit MEGAVOLT = SI.VOLT.withPrefix(SI.MEGA);
+	
+	public static final LinearUnit KILOOHM = SI.OHM.withPrefix(SI.KILO);
+	public static final LinearUnit MEGAOHM = SI.OHM.withPrefix(SI.MEGA);
+	
+	// sets of prefixes
+	public static final Set<UnitPrefix> ALL_PREFIXES = Set.of(DEKA, HECTO, KILO,
+			MEGA, GIGA, TERA, PETA, EXA, ZETTA, YOTTA, DECI, CENTI, MILLI, MICRO,
+			NANO, PICO, FEMTO, ATTO, ZEPTO, YOCTO, KIBI, MEBI, GIBI, TEBI, PEBI,
+			EXBI);
+	
+	public static final Set<UnitPrefix> DECIMAL_PREFIXES = Set.of(DEKA, HECTO,
+			KILO, MEGA, GIGA, TERA, PETA, EXA, ZETTA, YOTTA, DECI, CENTI, MILLI,
+			MICRO, NANO, PICO, FEMTO, ATTO, ZEPTO, YOCTO);
+	public static final Set<UnitPrefix> THOUSAND_PREFIXES = Set.of(KILO, MEGA,
+			GIGA, TERA, PETA, EXA, ZETTA, YOTTA, MILLI, MICRO, NANO, PICO, FEMTO,
+			ATTO, ZEPTO, YOCTO);
+	public static final Set<UnitPrefix> MAGNIFYING_PREFIXES = Set.of(DEKA, HECTO,
+			KILO, MEGA, GIGA, TERA, PETA, EXA, ZETTA, YOTTA, KIBI, MEBI, GIBI,
+			TEBI, PEBI, EXBI);
+	public static final Set<UnitPrefix> REDUCING_PREFIXES = Set.of(DECI, CENTI,
+			MILLI, MICRO, NANO, PICO, FEMTO, ATTO, ZEPTO, YOCTO);
+	
+	// You may NOT get SI instances!
+	private SI() {
+		throw new AssertionError();
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/USCustomary.java b/src/main/java/org/unitConverter/unit/USCustomary.java
new file mode 100644
index 0000000..1c4bcfe
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/USCustomary.java
@@ -0,0 +1,135 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+/**
+ * A static utility class that contains units in the US Customary system.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-21
+ */
+public final class USCustomary {
+	/**
+	 * US Customary units that measure area
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Area {
+		public static final LinearUnit SQUARE_SURVEY_FOOT = Length.SURVEY_FOOT.times(Length.SURVEY_FOOT);
+		public static final LinearUnit SQUARE_CHAIN = Length.SURVEY_CHAIN.times(Length.SURVEY_CHAIN);
+		public static final LinearUnit ACRE = Length.SURVEY_CHAIN.times(Length.SURVEY_FURLONG);
+		public static final LinearUnit SECTION = Length.SURVEY_MILE.times(Length.SURVEY_MILE);
+		public static final LinearUnit SURVEY_TOWNSHIP = SECTION.times(36);
+	}
+
+	/**
+	 * US Customary units that measure length
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-10-28
+	 */
+	public static final class Length {
+		public static final LinearUnit FOOT = BritishImperial.Length.FOOT;
+		public static final LinearUnit INCH = BritishImperial.Length.INCH;
+		public static final LinearUnit HAND = INCH.times(4);
+		public static final LinearUnit PICA = INCH.dividedBy(6);
+		public static final LinearUnit POINT = PICA.dividedBy(12);
+		public static final LinearUnit YARD = BritishImperial.Length.YARD;
+		public static final LinearUnit MILE = BritishImperial.Length.MILE;
+
+		public static final LinearUnit SURVEY_FOOT = SI.METRE.times(1200.0 / 3937.0);
+		public static final LinearUnit SURVEY_LINK = SURVEY_FOOT.times(33.0 / 50.0);
+		public static final LinearUnit SURVEY_ROD = SURVEY_FOOT.times(16.5);
+		public static final LinearUnit SURVEY_CHAIN = SURVEY_ROD.times(4);
+		public static final LinearUnit SURVEY_FURLONG = SURVEY_CHAIN.times(10);
+		public static final LinearUnit SURVEY_MILE = SURVEY_FURLONG.times(8);
+		public static final LinearUnit SURVEY_LEAGUE = SURVEY_MILE.times(3);
+
+		public static final LinearUnit NAUTICAL_MILE = BritishImperial.Length.NAUTICAL_MILE;
+		public static final LinearUnit FATHOM = YARD.times(2);
+		public static final LinearUnit CABLE = FATHOM.times(120);
+	}
+
+	/**
+	 * mass units
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Mass {
+		public static final LinearUnit GRAIN = BritishImperial.Mass.GRAIN;
+		public static final LinearUnit DRAM = BritishImperial.Mass.DRACHM;
+		public static final LinearUnit OUNCE = BritishImperial.Mass.OUNCE;
+		public static final LinearUnit POUND = BritishImperial.Mass.POUND;
+		public static final LinearUnit HUNDREDWEIGHT = POUND.times(100);
+		public static final LinearUnit SHORT_TON = HUNDREDWEIGHT.times(20);
+
+		// troy system for precious metals
+		public static final LinearUnit PENNYWEIGHT = GRAIN.times(24);
+		public static final LinearUnit TROY_OUNCE = PENNYWEIGHT.times(20);
+		public static final LinearUnit TROY_POUND = TROY_OUNCE.times(12);
+	}
+
+	/**
+	 * Volume units
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-11-08
+	 */
+	public static final class Volume {
+		public static final LinearUnit CUBIC_INCH = Length.INCH.toExponent(3);
+		public static final LinearUnit CUBIC_FOOT = Length.FOOT.toExponent(3);
+		public static final LinearUnit CUBIC_YARD = Length.YARD.toExponent(3);
+		public static final LinearUnit ACRE_FOOT = Area.ACRE.times(Length.FOOT);
+
+		public static final LinearUnit MINIM = SI.LITRE.withPrefix(SI.MICRO).times(61.611519921875);
+		public static final LinearUnit FLUID_DRAM = MINIM.times(60);
+		public static final LinearUnit TEASPOON = MINIM.times(80);
+		public static final LinearUnit TABLESPOON = TEASPOON.times(3);
+		public static final LinearUnit FLUID_OUNCE = TABLESPOON.times(2);
+		public static final LinearUnit SHOT = TABLESPOON.times(3);
+		public static final LinearUnit GILL = FLUID_OUNCE.times(4);
+		public static final LinearUnit CUP = GILL.times(2);
+		public static final LinearUnit PINT = CUP.times(2);
+		public static final LinearUnit QUART = PINT.times(2);
+		public static final LinearUnit GALLON = QUART.times(4);
+		public static final LinearUnit BARREL = GALLON.times(31.5);
+		public static final LinearUnit OIL_BARREL = GALLON.times(42);
+		public static final LinearUnit HOGSHEAD = GALLON.times(63);
+
+		public static final LinearUnit DRY_PINT = SI.LITRE.times(0.5506104713575);
+		public static final LinearUnit DRY_QUART = DRY_PINT.times(2);
+		public static final LinearUnit DRY_GALLON = DRY_QUART.times(4);
+		public static final LinearUnit PECK = DRY_GALLON.times(2);
+		public static final LinearUnit BUSHEL = PECK.times(4);
+		public static final LinearUnit DRY_BARREL = CUBIC_INCH.times(7056);
+	}
+
+	public static final LinearUnit OUNCE_FORCE = BritishImperial.OUNCE_FORCE;
+	public static final LinearUnit POUND_FORCE = BritishImperial.POUND_FORCE;
+
+	public static final LinearUnit BRITISH_THERMAL_UNIT = BritishImperial.BRITISH_THERMAL_UNIT;
+	public static final LinearUnit CALORIE = BritishImperial.CALORIE;
+	public static final LinearUnit KILOCALORIE = BritishImperial.KILOCALORIE;
+	public static final LinearUnit FOOT_POUND = POUND_FORCE.times(Length.FOOT);
+
+	public static final LinearUnit HORSEPOWER = Length.FOOT.times(POUND_FORCE).dividedBy(SI.MINUTE).times(33000);
+	public static final LinearUnit POUND_PER_SQUARE_INCH = POUND_FORCE.dividedBy(Length.INCH.toExponent(2));
+
+	public static final Unit FAHRENHEIT = BritishImperial.FAHRENHEIT;
+}
diff --git a/src/main/java/org/unitConverter/unit/Unit.java b/src/main/java/org/unitConverter/unit/Unit.java
new file mode 100644
index 0000000..0a3298f
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/Unit.java
@@ -0,0 +1,377 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Map;
+import java.util.Objects;
+import java.util.Set;
+import java.util.function.DoubleUnaryOperator;
+
+import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * A unit that is composed of base units.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public abstract class Unit implements Nameable {
+	/**
+	 * Returns a unit from its base and the functions it uses to convert to and
+	 * from its base.
+	 * 
+	 * <p>
+	 * For example, to get a unit representing the degree Celsius, the following
+	 * code can be used:
+	 * 
+	 * {@code Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK - 273.15, tempC -> tempC + 273.15);}
+	 * </p>
+	 * 
+	 * @param base          unit's base
+	 * @param converterFrom function that accepts a value expressed in the unit's
+	 *                      base and returns that value expressed in this unit.
+	 * @param converterTo   function that accepts a value expressed in the unit
+	 *                      and returns that value expressed in the unit's base.
+	 * @return a unit that uses the provided functions to convert.
+	 * @since 2019-05-22
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final Unit fromConversionFunctions(
+			final ObjectProduct<BaseUnit> base,
+			final DoubleUnaryOperator converterFrom,
+			final DoubleUnaryOperator converterTo) {
+		return new FunctionalUnit(base, converterFrom, converterTo);
+	}
+	
+	/**
+	 * Returns a unit from its base and the functions it uses to convert to and
+	 * from its base.
+	 * 
+	 * <p>
+	 * For example, to get a unit representing the degree Celsius, the following
+	 * code can be used:
+	 * 
+	 * {@code Unit.fromConversionFunctions(SI.KELVIN, tempK -> tempK - 273.15, tempC -> tempC + 273.15);}
+	 * </p>
+	 * 
+	 * @param base          unit's base
+	 * @param converterFrom function that accepts a value expressed in the unit's
+	 *                      base and returns that value expressed in this unit.
+	 * @param converterTo   function that accepts a value expressed in the unit
+	 *                      and returns that value expressed in the unit's base.
+	 * @param ns            names and symbol of unit
+	 * @return a unit that uses the provided functions to convert.
+	 * @since 2019-05-22
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final Unit fromConversionFunctions(
+			final ObjectProduct<BaseUnit> base,
+			final DoubleUnaryOperator converterFrom,
+			final DoubleUnaryOperator converterTo, final NameSymbol ns) {
+		return new FunctionalUnit(base, converterFrom, converterTo, ns);
+	}
+	
+	/**
+	 * The combination of units that this unit is based on.
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final ObjectProduct<BaseUnit> unitBase;
+	
+	/**
+	 * This unit's name(s) and symbol
+	 * 
+	 * @since 2020-09-07
+	 */
+	private final NameSymbol nameSymbol;
+	
+	/**
+	 * Cache storing the result of getDimension()
+	 *
+	 * @since 2019-10-16
+	 */
+	private transient ObjectProduct<BaseDimension> dimension = null;
+	
+	/**
+	 * Creates the {@code Unit}.
+	 * 
+	 * @param unitBase base of unit
+	 * @param ns       names and symbol of unit
+	 * @since 2019-10-16
+	 * @throws NullPointerException if unitBase or ns is null
+	 */
+	Unit(ObjectProduct<BaseUnit> unitBase, NameSymbol ns) {
+		this.unitBase = Objects.requireNonNull(unitBase,
+				"unitBase may not be null");
+		this.nameSymbol = Objects.requireNonNull(ns, "ns may not be null");
+	}
+	
+	/**
+	 * A constructor that constructs {@code BaseUnit} instances.
+	 * 
+	 * @since 2019-10-16
+	 */
+	Unit(final String primaryName, final String symbol,
+			final Set<String> otherNames) {
+		if (this instanceof BaseUnit) {
+			this.unitBase = ObjectProduct.oneOf((BaseUnit) this);
+		} else
+			throw new AssertionError();
+		this.nameSymbol = NameSymbol.of(primaryName, symbol,
+				new HashSet<>(otherNames));
+	}
+	
+	/**
+	 * @return this unit as a {@link Unitlike}
+	 * @since 2020-09-07
+	 */
+	public final Unitlike<Double> asUnitlike() {
+		return Unitlike.fromConversionFunctions(this.getBase(),
+				this::convertFromBase, this::convertToBase, this.getNameSymbol());
+	}
+	
+	/**
+	 * Checks if a value expressed in this unit can be converted to a value
+	 * expressed in {@code other}
+	 * 
+	 * @param other unit or unitlike form to test with
+	 * @return true if they are compatible
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 * @throws NullPointerException if other is null
+	 */
+	public final boolean canConvertTo(final Unit other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		return Objects.equals(this.getBase(), other.getBase());
+	}
+	
+	/**
+	 * Checks if a value expressed in this unit can be converted to a value
+	 * expressed in {@code other}
+	 * 
+	 * @param other unit or unitlike form to test with
+	 * @return true if they are compatible
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 * @throws NullPointerException if other is null
+	 */
+	public final <W> boolean canConvertTo(final Unitlike<W> other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		return Objects.equals(this.getBase(), other.getBase());
+	}
+	
+	/**
+	 * Converts from a value expressed in this unit's base unit to a value
+	 * expressed in this unit.
+	 * <p>
+	 * This must be the inverse of {@code convertToBase}, so
+	 * {@code convertFromBase(convertToBase(value))} must be equal to
+	 * {@code value} for any value, ignoring precision loss by roundoff error.
+	 * </p>
+	 * <p>
+	 * If this unit <i>is</i> a base unit, this method should return
+	 * {@code value}.
+	 * </p>
+	 * 
+	 * @implSpec This method is used by {@link #convertTo}, and its behaviour
+	 *           affects the behaviour of {@code convertTo}.
+	 * 
+	 * @param value value expressed in <b>base</b> unit
+	 * @return value expressed in <b>this</b> unit
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	protected abstract double convertFromBase(double value);
+	
+	/**
+	 * Converts a value expressed in this unit to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If unit conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 
+	 * @param other unit to convert to
+	 * @param value value to convert
+	 * @return converted value
+	 * @since 2019-05-22
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unit (as tested by
+	 *                                  {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final double convertTo(final Unit other, final double value) {
+		Objects.requireNonNull(other, "other must not be null.");
+		if (this.canConvertTo(other))
+			return other.convertFromBase(this.convertToBase(value));
+		else
+			throw new IllegalArgumentException(
+					String.format("Cannot convert from %s to %s.", this, other));
+	}
+	
+	/**
+	 * Converts a value expressed in this unit to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 				
+	 * @param other unitlike form to convert to
+	 * @param value value to convert
+	 * @param <W>   type of value to convert to
+	 * @return converted value
+	 * @since 2020-09-07
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unit (as tested by
+	 *                                  {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final <W> W convertTo(final Unitlike<W> other, final double value) {
+		Objects.requireNonNull(other, "other must not be null.");
+		if (this.canConvertTo(other))
+			return other.convertFromBase(this.convertToBase(value));
+		else
+			throw new IllegalArgumentException(
+					String.format("Cannot convert from %s to %s.", this, other));
+	}
+	
+	/**
+	 * Converts from a value expressed in this unit to a value expressed in this
+	 * unit's base unit.
+	 * <p>
+	 * This must be the inverse of {@code convertFromBase}, so
+	 * {@code convertToBase(convertFromBase(value))} must be equal to
+	 * {@code value} for any value, ignoring precision loss by roundoff error.
+	 * </p>
+	 * <p>
+	 * If this unit <i>is</i> a base unit, this method should return
+	 * {@code value}.
+	 * </p>
+	 * 
+	 * @implSpec This method is used by {@link #convertTo}, and its behaviour
+	 *           affects the behaviour of {@code convertTo}.
+	 * 
+	 * @param value value expressed in <b>this</b> unit
+	 * @return value expressed in <b>base</b> unit
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	protected abstract double convertToBase(double value);
+	
+	/**
+	 * @return combination of units that this unit is based on
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public final ObjectProduct<BaseUnit> getBase() {
+		return this.unitBase;
+	}
+	
+	/**
+	 * @return dimension measured by this unit
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public final ObjectProduct<BaseDimension> getDimension() {
+		if (this.dimension == null) {
+			final Map<BaseUnit, Integer> mapping = this.unitBase.exponentMap();
+			final Map<BaseDimension, Integer> dimensionMap = new HashMap<>();
+			
+			for (final BaseUnit key : mapping.keySet()) {
+				dimensionMap.put(key.getBaseDimension(), mapping.get(key));
+			}
+			
+			this.dimension = ObjectProduct.fromExponentMapping(dimensionMap);
+		}
+		return this.dimension;
+	}
+	
+	/**
+	 * @return the nameSymbol
+	 * @since 2020-09-07
+	 */
+	@Override
+	public final NameSymbol getNameSymbol() {
+		return this.nameSymbol;
+	}
+	
+	/**
+	 * Returns true iff this unit is metric.
+	 * <p>
+	 * "Metric" is defined by three conditions:
+	 * <ul>
+	 * <li>Must be an instance of {@link LinearUnit}.</li>
+	 * <li>Must be based on the SI base units (as determined by getBase())</li>
+	 * <li>The conversion factor must be a power of 10.</li>
+	 * </ul>
+	 * <p>
+	 * Note that this definition excludes some units that many would consider
+	 * "metric", such as the degree Celsius (fails the first condition),
+	 * calories, minutes and hours (fail the third condition).
+	 * <p>
+	 * All SI units (as designated by the BIPM) except the degree Celsius are
+	 * considered "metric" by this definition.
+	 * 
+	 * @since 2020-08-27
+	 */
+	public final boolean isMetric() {
+		// first condition - check that it is a linear unit
+		if (!(this instanceof LinearUnit))
+			return false;
+		final LinearUnit linear = (LinearUnit) this;
+		
+		// second condition - check that
+		for (final BaseUnit b : linear.getBase().getBaseSet()) {
+			if (!SI.BaseUnits.BASE_UNITS.contains(b))
+				return false;
+		}
+		
+		// third condition - check that conversion factor is a power of 10
+		return DecimalComparison
+				.equals(Math.log10(linear.getConversionFactor()) % 1.0, 0);
+	}
+	
+	@Override
+	public String toString() {
+		return this.getPrimaryName().orElse("Unnamed unit")
+				+ (this.getSymbol().isPresent()
+						? String.format(" (%s)", this.getSymbol().get())
+						: "")
+				+ ", derived from "
+				+ this.getBase().toString(u -> u.getSymbol().get())
+				+ (this.getOtherNames().isEmpty() ? ""
+						: ", also called " + String.join(", ", this.getOtherNames()));
+	}
+	
+	/**
+	 * @param ns name(s) and symbol to use
+	 * @return a copy of this unit with provided name(s) and symbol
+	 * @since 2019-10-21
+	 * @throws NullPointerException if ns is null
+	 */
+	public Unit withName(final NameSymbol ns) {
+		return fromConversionFunctions(this.getBase(), this::convertFromBase,
+				this::convertToBase,
+				Objects.requireNonNull(ns, "ns must not be null."));
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/UnitDatabase.java b/src/main/java/org/unitConverter/unit/UnitDatabase.java
new file mode 100644
index 0000000..6322fef
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/UnitDatabase.java
@@ -0,0 +1,2025 @@
+/**
+ * Copyright (C) 2018 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.io.FileNotFoundException;
+import java.io.IOException;
+import java.io.InputStream;
+import java.math.BigDecimal;
+import java.nio.file.Files;
+import java.nio.file.Path;
+import java.util.AbstractSet;
+import java.util.ArrayList;
+import java.util.Arrays;
+import java.util.Collection;
+import java.util.Collections;
+import java.util.HashMap;
+import java.util.HashSet;
+import java.util.Iterator;
+import java.util.List;
+import java.util.Map;
+import java.util.Map.Entry;
+import java.util.NoSuchElementException;
+import java.util.Objects;
+import java.util.Scanner;
+import java.util.Set;
+import java.util.function.BiFunction;
+import java.util.function.Function;
+import java.util.function.Predicate;
+import java.util.regex.Matcher;
+import java.util.regex.Pattern;
+
+import org.unitConverter.math.ConditionalExistenceCollections;
+import org.unitConverter.math.DecimalComparison;
+import org.unitConverter.math.ExpressionParser;
+import org.unitConverter.math.ObjectProduct;
+import org.unitConverter.math.UncertainDouble;
+
+/**
+ * A database of units, prefixes and dimensions, and their names.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-01-07
+ * @since v0.1.0
+ */
+public final class UnitDatabase {
+	/**
+	 * A map for units that allows the use of prefixes.
+	 * <p>
+	 * As this map implementation is intended to be used as a sort of "augmented
+	 * view" of a unit and prefix map, it is unmodifiable but instead reflects
+	 * the changes to the maps passed into it. Do not edit this map, instead edit
+	 * the maps that were passed in during construction.
+	 * </p>
+	 * <p>
+	 * The rules for applying prefixes onto units are the following:
+	 * <ul>
+	 * <li>Prefixes can only be applied to linear units.</li>
+	 * <li>Before attempting to search for prefixes in a unit name, this map will
+	 * first search for a unit name. So, if there are two units, "B" and "AB",
+	 * and a prefix "A", this map will favour the unit "AB" over the unit "B"
+	 * with the prefix "A", even though they have the same string.</li>
+	 * <li>Longer prefixes are preferred to shorter prefixes. So, if you have
+	 * units "BC" and "C", and prefixes "AB" and "A", inputting "ABC" will return
+	 * the unit "C" with the prefix "AB", not "BC" with the prefix "A".</li>
+	 * </ul>
+	 * </p>
+	 * <p>
+	 * This map is infinite in size if there is at least one unit and at least
+	 * one prefix. If it is infinite, some operations that only work with finite
+	 * collections, like converting name/entry sets to arrays, will throw an
+	 * {@code IllegalStateException}.
+	 * </p>
+	 * <p>
+	 * Because of ambiguities between prefixes (i.e. kilokilo = mega),
+	 * {@link #containsValue} and {@link #values()} currently ignore prefixes.
+	 * </p>
+	 * 
+	 * @author Adrien Hopkins
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private static final class PrefixedUnitMap implements Map<String, Unit> {
+		/**
+		 * The class used for entry sets.
+		 * 
+		 * <p>
+		 * If the map that created this set is infinite in size (has at least one
+		 * unit and at least one prefix), this set is infinite as well. If this
+		 * set is infinite in size, {@link #toArray} will fail with a
+		 * {@code IllegalStateException} instead of creating an infinite-sized
+		 * array.
+		 * </p>
+		 * 
+		 * @author Adrien Hopkins
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private static final class PrefixedUnitEntrySet
+				extends AbstractSet<Map.Entry<String, Unit>> {
+			/**
+			 * The entry for this set.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitEntry
+					implements Entry<String, Unit> {
+				private final String key;
+				private final Unit value;
+				
+				/**
+				 * Creates the {@code PrefixedUnitEntry}.
+				 * 
+				 * @param key   key
+				 * @param value value
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitEntry(final String key, final Unit value) {
+					this.key = key;
+					this.value = value;
+				}
+				
+				/**
+				 * @since 2019-05-03
+				 */
+				@Override
+				public boolean equals(final Object o) {
+					if (!(o instanceof Map.Entry))
+						return false;
+					final Map.Entry<?, ?> other = (Map.Entry<?, ?>) o;
+					return Objects.equals(this.getKey(), other.getKey())
+							&& Objects.equals(this.getValue(), other.getValue());
+				}
+				
+				@Override
+				public String getKey() {
+					return this.key;
+				}
+				
+				@Override
+				public Unit getValue() {
+					return this.value;
+				}
+				
+				/**
+				 * @since 2019-05-03
+				 */
+				@Override
+				public int hashCode() {
+					return (this.getKey() == null ? 0 : this.getKey().hashCode())
+							^ (this.getValue() == null ? 0
+									: this.getValue().hashCode());
+				}
+				
+				@Override
+				public Unit setValue(final Unit value) {
+					throw new UnsupportedOperationException(
+							"Cannot set value in an immutable entry");
+				}
+				
+				/**
+				 * Returns a string representation of the entry. The format of the
+				 * string is the string representation of the key, then the equals
+				 * ({@code =}) character, then the string representation of the
+				 * value.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return this.getKey() + "=" + this.getValue();
+				}
+			}
+			
+			/**
+			 * An iterator that iterates over the units of a
+			 * {@code PrefixedUnitNameSet}.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitEntryIterator
+					implements Iterator<Entry<String, Unit>> {
+				// position in the unit list
+				private int unitNamePosition = 0;
+				// the indices of the prefixes attached to the current unit
+				private final List<Integer> prefixCoordinates = new ArrayList<>();
+				
+				// values from the unit entry set
+				private final Map<String, Unit> map;
+				private transient final List<String> unitNames;
+				private transient final List<String> prefixNames;
+				
+				/**
+				 * Creates the
+				 * {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitEntryIterator(final PrefixedUnitMap map) {
+					this.map = map;
+					this.unitNames = new ArrayList<>(map.units.keySet());
+					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
+				}
+				
+				/**
+				 * @return current unit name
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private String getCurrentUnitName() {
+					final StringBuilder unitName = new StringBuilder();
+					for (final int i : this.prefixCoordinates) {
+						unitName.append(this.prefixNames.get(i));
+					}
+					unitName.append(this.unitNames.get(this.unitNamePosition));
+					
+					return unitName.toString();
+				}
+				
+				@Override
+				public boolean hasNext() {
+					if (this.unitNames.isEmpty())
+						return false;
+					else {
+						if (this.prefixNames.isEmpty())
+							return this.unitNamePosition >= this.unitNames.size() - 1;
+						else
+							return true;
+					}
+				}
+				
+				/**
+				 * Changes this iterator's position to the next available one.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private void incrementPosition() {
+					this.unitNamePosition++;
+					
+					if (this.unitNamePosition >= this.unitNames.size()) {
+						// we have used all of our units, go to a different prefix
+						this.unitNamePosition = 0;
+						
+						// if the prefix coordinates are empty, then set it to [0]
+						if (this.prefixCoordinates.isEmpty()) {
+							this.prefixCoordinates.add(0, 0);
+						} else {
+							// get the prefix coordinate to increment, then increment
+							int i = this.prefixCoordinates.size() - 1;
+							this.prefixCoordinates.set(i,
+									this.prefixCoordinates.get(i) + 1);
+							
+							// fix any carrying errors
+							while (i >= 0 && this.prefixCoordinates
+									.get(i) >= this.prefixNames.size()) {
+								// carry over
+								this.prefixCoordinates.set(i--, 0); // null and
+																				// decrement at the
+																				// same time
+								
+								if (i < 0) { // we need to add a new coordinate
+									this.prefixCoordinates.add(0, 0);
+								} else { // increment an existing one
+									this.prefixCoordinates.set(i,
+											this.prefixCoordinates.get(i) + 1);
+								}
+							}
+						}
+					}
+				}
+				
+				@Override
+				public Entry<String, Unit> next() {
+					// get next element
+					final Entry<String, Unit> nextEntry = this.peek();
+					
+					// iterate to next position
+					this.incrementPosition();
+					
+					return nextEntry;
+				}
+				
+				/**
+				 * @return the next element in the iterator, without iterating over
+				 *         it
+				 * @since 2019-05-03
+				 */
+				private Entry<String, Unit> peek() {
+					if (!this.hasNext())
+						throw new NoSuchElementException("No units left!");
+						
+					// if I have prefixes, ensure I'm not using a nonlinear unit
+					// since all of the unprefixed stuff is done, just remove
+					// nonlinear units
+					if (!this.prefixCoordinates.isEmpty()) {
+						while (this.unitNamePosition < this.unitNames.size()
+								&& !(this.map.get(this.unitNames.get(
+										this.unitNamePosition)) instanceof LinearUnit)) {
+							this.unitNames.remove(this.unitNamePosition);
+						}
+					}
+					
+					final String nextName = this.getCurrentUnitName();
+					
+					return new PrefixedUnitEntry(nextName, this.map.get(nextName));
+				}
+				
+				/**
+				 * Returns a string representation of the object. The exact details
+				 * of the representation are unspecified and subject to change.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return String.format(
+							"Iterator iterating over name-unit entries; next value is \"%s\"",
+							this.peek());
+				}
+			}
+			
+			// the map that created this set
+			private final PrefixedUnitMap map;
+			
+			/**
+			 * Creates the {@code PrefixedUnitNameSet}.
+			 * 
+			 * @param map map that created this set
+			 * @since 2019-04-13
+			 * @since v0.2.0
+			 */
+			public PrefixedUnitEntrySet(final PrefixedUnitMap map) {
+				this.map = map;
+			}
+			
+			@Override
+			public boolean add(final Map.Entry<String, Unit> e) {
+				throw new UnsupportedOperationException(
+						"Cannot add to an immutable set");
+			}
+			
+			@Override
+			public boolean addAll(
+					final Collection<? extends Map.Entry<String, Unit>> c) {
+				throw new UnsupportedOperationException(
+						"Cannot add to an immutable set");
+			}
+			
+			@Override
+			public void clear() {
+				throw new UnsupportedOperationException(
+						"Cannot clear an immutable set");
+			}
+			
+			@Override
+			public boolean contains(final Object o) {
+				// get the entry
+				final Entry<String, Unit> entry;
+				
+				try {
+					// This is OK because I'm in a try-catch block, catching the
+					// exact exception that would be thrown.
+					@SuppressWarnings("unchecked")
+					final Entry<String, Unit> tempEntry = (Entry<String, Unit>) o;
+					entry = tempEntry;
+				} catch (final ClassCastException e) {
+					throw new IllegalArgumentException(
+							"Attempted to test for an entry using a non-entry.");
+				}
+				
+				return this.map.containsKey(entry.getKey())
+						&& this.map.get(entry.getKey()).equals(entry.getValue());
+			}
+			
+			@Override
+			public boolean containsAll(final Collection<?> c) {
+				for (final Object o : c)
+					if (!this.contains(o))
+						return false;
+				return true;
+			}
+			
+			@Override
+			public boolean isEmpty() {
+				return this.map.isEmpty();
+			}
+			
+			@Override
+			public Iterator<Entry<String, Unit>> iterator() {
+				return new PrefixedUnitEntryIterator(this.map);
+			}
+			
+			@Override
+			public boolean remove(final Object o) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean removeAll(final Collection<?> c) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean removeIf(
+					final Predicate<? super Entry<String, Unit>> filter) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean retainAll(final Collection<?> c) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public int size() {
+				if (this.map.units.isEmpty())
+					return 0;
+				else {
+					if (this.map.prefixes.isEmpty())
+						return this.map.units.size();
+					else
+						// infinite set
+						return Integer.MAX_VALUE;
+				}
+			}
+			
+			/**
+			 * @throws IllegalStateException if the set is infinite in size
+			 */
+			@Override
+			public Object[] toArray() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray();
+				else
+					// infinite set
+					throw new IllegalStateException(
+							"Cannot make an infinite set into an array.");
+			}
+			
+			/**
+			 * @throws IllegalStateException if the set is infinite in size
+			 */
+			@Override
+			public <T> T[] toArray(final T[] a) {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray(a);
+				else
+					// infinite set
+					throw new IllegalStateException(
+							"Cannot make an infinite set into an array.");
+			}
+			
+			@Override
+			public String toString() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toString();
+				else
+					return String.format(
+							"Infinite set of name-unit entries created from units %s and prefixes %s",
+							this.map.units, this.map.prefixes);
+			}
+		}
+		
+		/**
+		 * The class used for unit name sets.
+		 * 
+		 * <p>
+		 * If the map that created this set is infinite in size (has at least one
+		 * unit and at least one prefix), this set is infinite as well. If this
+		 * set is infinite in size, {@link #toArray} will fail with a
+		 * {@code IllegalStateException} instead of creating an infinite-sized
+		 * array.
+		 * </p>
+		 * 
+		 * @author Adrien Hopkins
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private static final class PrefixedUnitNameSet
+				extends AbstractSet<String> {
+			/**
+			 * An iterator that iterates over the units of a
+			 * {@code PrefixedUnitNameSet}.
+			 * 
+			 * @author Adrien Hopkins
+			 * @since 2019-04-14
+			 * @since v0.2.0
+			 */
+			private static final class PrefixedUnitNameIterator
+					implements Iterator<String> {
+				// position in the unit list
+				private int unitNamePosition = 0;
+				// the indices of the prefixes attached to the current unit
+				private final List<Integer> prefixCoordinates = new ArrayList<>();
+				
+				// values from the unit name set
+				private final Map<String, Unit> map;
+				private transient final List<String> unitNames;
+				private transient final List<String> prefixNames;
+				
+				/**
+				 * Creates the
+				 * {@code UnitsDatabase.PrefixedUnitMap.PrefixedUnitNameSet.PrefixedUnitNameIterator}.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				public PrefixedUnitNameIterator(final PrefixedUnitMap map) {
+					this.map = map;
+					this.unitNames = new ArrayList<>(map.units.keySet());
+					this.prefixNames = new ArrayList<>(map.prefixes.keySet());
+				}
+				
+				/**
+				 * @return current unit name
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private String getCurrentUnitName() {
+					final StringBuilder unitName = new StringBuilder();
+					for (final int i : this.prefixCoordinates) {
+						unitName.append(this.prefixNames.get(i));
+					}
+					unitName.append(this.unitNames.get(this.unitNamePosition));
+					
+					return unitName.toString();
+				}
+				
+				@Override
+				public boolean hasNext() {
+					if (this.unitNames.isEmpty())
+						return false;
+					else {
+						if (this.prefixNames.isEmpty())
+							return this.unitNamePosition >= this.unitNames.size() - 1;
+						else
+							return true;
+					}
+				}
+				
+				/**
+				 * Changes this iterator's position to the next available one.
+				 * 
+				 * @since 2019-04-14
+				 * @since v0.2.0
+				 */
+				private void incrementPosition() {
+					this.unitNamePosition++;
+					
+					if (this.unitNamePosition >= this.unitNames.size()) {
+						// we have used all of our units, go to a different prefix
+						this.unitNamePosition = 0;
+						
+						// if the prefix coordinates are empty, then set it to [0]
+						if (this.prefixCoordinates.isEmpty()) {
+							this.prefixCoordinates.add(0, 0);
+						} else {
+							// get the prefix coordinate to increment, then increment
+							int i = this.prefixCoordinates.size() - 1;
+							this.prefixCoordinates.set(i,
+									this.prefixCoordinates.get(i) + 1);
+							
+							// fix any carrying errors
+							while (i >= 0 && this.prefixCoordinates
+									.get(i) >= this.prefixNames.size()) {
+								// carry over
+								this.prefixCoordinates.set(i--, 0); // null and
+																				// decrement at the
+																				// same time
+								
+								if (i < 0) { // we need to add a new coordinate
+									this.prefixCoordinates.add(0, 0);
+								} else { // increment an existing one
+									this.prefixCoordinates.set(i,
+											this.prefixCoordinates.get(i) + 1);
+								}
+							}
+						}
+					}
+				}
+				
+				@Override
+				public String next() {
+					final String nextName = this.peek();
+					
+					this.incrementPosition();
+					
+					return nextName;
+				}
+				
+				/**
+				 * @return the next element in the iterator, without iterating over
+				 *         it
+				 * @since 2019-05-03
+				 */
+				private String peek() {
+					if (!this.hasNext())
+						throw new NoSuchElementException("No units left!");
+					// if I have prefixes, ensure I'm not using a nonlinear unit
+					// since all of the unprefixed stuff is done, just remove
+					// nonlinear units
+					if (!this.prefixCoordinates.isEmpty()) {
+						while (this.unitNamePosition < this.unitNames.size()
+								&& !(this.map.get(this.unitNames.get(
+										this.unitNamePosition)) instanceof LinearUnit)) {
+							this.unitNames.remove(this.unitNamePosition);
+						}
+					}
+					
+					return this.getCurrentUnitName();
+				}
+				
+				/**
+				 * Returns a string representation of the object. The exact details
+				 * of the representation are unspecified and subject to change.
+				 * 
+				 * @since 2019-05-03
+				 */
+				@Override
+				public String toString() {
+					return String.format(
+							"Iterator iterating over unit names; next value is \"%s\"",
+							this.peek());
+				}
+			}
+			
+			// the map that created this set
+			private final PrefixedUnitMap map;
+			
+			/**
+			 * Creates the {@code PrefixedUnitNameSet}.
+			 * 
+			 * @param map map that created this set
+			 * @since 2019-04-13
+			 * @since v0.2.0
+			 */
+			public PrefixedUnitNameSet(final PrefixedUnitMap map) {
+				this.map = map;
+			}
+			
+			@Override
+			public boolean add(final String e) {
+				throw new UnsupportedOperationException(
+						"Cannot add to an immutable set");
+			}
+			
+			@Override
+			public boolean addAll(final Collection<? extends String> c) {
+				throw new UnsupportedOperationException(
+						"Cannot add to an immutable set");
+			}
+			
+			@Override
+			public void clear() {
+				throw new UnsupportedOperationException(
+						"Cannot clear an immutable set");
+			}
+			
+			@Override
+			public boolean contains(final Object o) {
+				return this.map.containsKey(o);
+			}
+			
+			@Override
+			public boolean containsAll(final Collection<?> c) {
+				for (final Object o : c)
+					if (!this.contains(o))
+						return false;
+				return true;
+			}
+			
+			@Override
+			public boolean isEmpty() {
+				return this.map.isEmpty();
+			}
+			
+			@Override
+			public Iterator<String> iterator() {
+				return new PrefixedUnitNameIterator(this.map);
+			}
+			
+			@Override
+			public boolean remove(final Object o) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean removeAll(final Collection<?> c) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean removeIf(final Predicate<? super String> filter) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public boolean retainAll(final Collection<?> c) {
+				throw new UnsupportedOperationException(
+						"Cannot remove from an immutable set");
+			}
+			
+			@Override
+			public int size() {
+				if (this.map.units.isEmpty())
+					return 0;
+				else {
+					if (this.map.prefixes.isEmpty())
+						return this.map.units.size();
+					else
+						// infinite set
+						return Integer.MAX_VALUE;
+				}
+			}
+			
+			/**
+			 * @throws IllegalStateException if the set is infinite in size
+			 */
+			@Override
+			public Object[] toArray() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray();
+				else
+					// infinite set
+					throw new IllegalStateException(
+							"Cannot make an infinite set into an array.");
+				
+			}
+			
+			/**
+			 * @throws IllegalStateException if the set is infinite in size
+			 */
+			@Override
+			public <T> T[] toArray(final T[] a) {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toArray(a);
+				else
+					// infinite set
+					throw new IllegalStateException(
+							"Cannot make an infinite set into an array.");
+			}
+			
+			@Override
+			public String toString() {
+				if (this.map.units.isEmpty() || this.map.prefixes.isEmpty())
+					return super.toString();
+				else
+					return String.format(
+							"Infinite set of name-unit entries created from units %s and prefixes %s",
+							this.map.units, this.map.prefixes);
+			}
+		}
+		
+		/**
+		 * The units stored in this collection, without prefixes.
+		 * 
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private final Map<String, Unit> units;
+		
+		/**
+		 * The available prefixes for use.
+		 * 
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		private final Map<String, UnitPrefix> prefixes;
+		
+		// caches
+		private transient Collection<Unit> values = null;
+		private transient Set<String> keySet = null;
+		private transient Set<Entry<String, Unit>> entrySet = null;
+		
+		/**
+		 * Creates the {@code PrefixedUnitMap}.
+		 * 
+		 * @param units    map mapping unit names to units
+		 * @param prefixes map mapping prefix names to prefixes
+		 * @since 2019-04-13
+		 * @since v0.2.0
+		 */
+		public PrefixedUnitMap(final Map<String, Unit> units,
+				final Map<String, UnitPrefix> prefixes) {
+			// I am making unmodifiable maps to ensure I don't accidentally make
+			// changes.
+			this.units = Collections.unmodifiableMap(units);
+			this.prefixes = Collections.unmodifiableMap(prefixes);
+		}
+		
+		@Override
+		public void clear() {
+			throw new UnsupportedOperationException(
+					"Cannot clear an immutable map");
+		}
+		
+		@Override
+		public Unit compute(final String key,
+				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException(
+					"Cannot edit an immutable map");
+		}
+		
+		@Override
+		public Unit computeIfAbsent(final String key,
+				final Function<? super String, ? extends Unit> mappingFunction) {
+			throw new UnsupportedOperationException(
+					"Cannot edit an immutable map");
+		}
+		
+		@Override
+		public Unit computeIfPresent(final String key,
+				final BiFunction<? super String, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException(
+					"Cannot edit an immutable map");
+		}
+		
+		@Override
+		public boolean containsKey(final Object key) {
+			// First, test if there is a unit with the key
+			if (this.units.containsKey(key))
+				return true;
+			
+			// Next, try to cast it to String
+			if (!(key instanceof String))
+				throw new IllegalArgumentException(
+						"Attempted to test for a unit using a non-string name.");
+			final String unitName = (String) key;
+			
+			// Then, look for the longest prefix that is attached to a valid unit
+			String longestPrefix = null;
+			int longestLength = 0;
+			
+			for (final String prefixName : this.prefixes.keySet()) {
+				// a prefix name is valid if:
+				// - it is prefixed (i.e. the unit name starts with it)
+				// - it is longer than the existing largest prefix (since I am
+				// looking for the longest valid prefix)
+				// - the part after the prefix is a valid unit name
+				// - the unit described that name is a linear unit (since only
+				// linear units can have prefixes)
+				if (unitName.startsWith(prefixName)
+						&& prefixName.length() > longestLength) {
+					final String rest = unitName.substring(prefixName.length());
+					if (this.containsKey(rest)
+							&& this.get(rest) instanceof LinearUnit) {
+						longestPrefix = prefixName;
+						longestLength = prefixName.length();
+					}
+				}
+			}
+			
+			return longestPrefix != null;
+		}
+		
+		/**
+		 * {@inheritDoc}
+		 * 
+		 * <p>
+		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this
+		 * method only tests for prefixless units.
+		 * </p>
+		 */
+		@Override
+		public boolean containsValue(final Object value) {
+			return this.units.containsValue(value);
+		}
+		
+		@Override
+		public Set<Entry<String, Unit>> entrySet() {
+			if (this.entrySet == null) {
+				this.entrySet = new PrefixedUnitEntrySet(this);
+			}
+			return this.entrySet;
+		}
+		
+		@Override
+		public Unit get(final Object key) {
+			// First, test if there is a unit with the key
+			if (this.units.containsKey(key))
+				return this.units.get(key);
+			
+			// Next, try to cast it to String
+			if (!(key instanceof String))
+				throw new IllegalArgumentException(
+						"Attempted to obtain a unit using a non-string name.");
+			final String unitName = (String) key;
+			
+			// Then, look for the longest prefix that is attached to a valid unit
+			String longestPrefix = null;
+			int longestLength = 0;
+			
+			for (final String prefixName : this.prefixes.keySet()) {
+				// a prefix name is valid if:
+				// - it is prefixed (i.e. the unit name starts with it)
+				// - it is longer than the existing largest prefix (since I am
+				// looking for the longest valid prefix)
+				// - the part after the prefix is a valid unit name
+				// - the unit described that name is a linear unit (since only
+				// linear units can have prefixes)
+				if (unitName.startsWith(prefixName)
+						&& prefixName.length() > longestLength) {
+					final String rest = unitName.substring(prefixName.length());
+					if (this.containsKey(rest)
+							&& this.get(rest) instanceof LinearUnit) {
+						longestPrefix = prefixName;
+						longestLength = prefixName.length();
+					}
+				}
+			}
+			
+			// if none found, returns null
+			if (longestPrefix == null)
+				return null;
+			else {
+				// get necessary data
+				final String rest = unitName.substring(longestLength);
+				// this cast will not fail because I verified that it would work
+				// before selecting this prefix
+				final LinearUnit unit = (LinearUnit) this.get(rest);
+				final UnitPrefix prefix = this.prefixes.get(longestPrefix);
+				
+				return unit.withPrefix(prefix);
+			}
+		}
+		
+		@Override
+		public boolean isEmpty() {
+			return this.units.isEmpty();
+		}
+		
+		@Override
+		public Set<String> keySet() {
+			if (this.keySet == null) {
+				this.keySet = new PrefixedUnitNameSet(this);
+			}
+			return this.keySet;
+		}
+		
+		@Override
+		public Unit merge(final String key, final Unit value,
+				final BiFunction<? super Unit, ? super Unit, ? extends Unit> remappingFunction) {
+			throw new UnsupportedOperationException(
+					"Cannot merge into an immutable map");
+		}
+		
+		@Override
+		public Unit put(final String key, final Unit value) {
+			throw new UnsupportedOperationException(
+					"Cannot add entries to an immutable map");
+		}
+		
+		@Override
+		public void putAll(final Map<? extends String, ? extends Unit> m) {
+			throw new UnsupportedOperationException(
+					"Cannot add entries to an immutable map");
+		}
+		
+		@Override
+		public Unit putIfAbsent(final String key, final Unit value) {
+			throw new UnsupportedOperationException(
+					"Cannot add entries to an immutable map");
+		}
+		
+		@Override
+		public Unit remove(final Object key) {
+			throw new UnsupportedOperationException(
+					"Cannot remove entries from an immutable map");
+		}
+		
+		@Override
+		public boolean remove(final Object key, final Object value) {
+			throw new UnsupportedOperationException(
+					"Cannot remove entries from an immutable map");
+		}
+		
+		@Override
+		public Unit replace(final String key, final Unit value) {
+			throw new UnsupportedOperationException(
+					"Cannot replace entries in an immutable map");
+		}
+		
+		@Override
+		public boolean replace(final String key, final Unit oldValue,
+				final Unit newValue) {
+			throw new UnsupportedOperationException(
+					"Cannot replace entries in an immutable map");
+		}
+		
+		@Override
+		public void replaceAll(
+				final BiFunction<? super String, ? super Unit, ? extends Unit> function) {
+			throw new UnsupportedOperationException(
+					"Cannot replace entries in an immutable map");
+		}
+		
+		@Override
+		public int size() {
+			if (this.units.isEmpty())
+				return 0;
+			else {
+				if (this.prefixes.isEmpty())
+					return this.units.size();
+				else
+					// infinite set
+					return Integer.MAX_VALUE;
+			}
+		}
+		
+		@Override
+		public String toString() {
+			if (this.units.isEmpty() || this.prefixes.isEmpty())
+				return super.toString();
+			else
+				return String.format(
+						"Infinite map of name-unit entries created from units %s and prefixes %s",
+						this.units, this.prefixes);
+		}
+		
+		/**
+		 * {@inheritDoc}
+		 * 
+		 * <p>
+		 * Because of ambiguities between prefixes (i.e. kilokilo = mega), this
+		 * method ignores prefixes.
+		 * </p>
+		 */
+		@Override
+		public Collection<Unit> values() {
+			if (this.values == null) {
+				this.values = Collections
+						.unmodifiableCollection(this.units.values());
+			}
+			return this.values;
+		}
+	}
+	
+	/**
+	 * Replacements done to *all* expression types
+	 */
+	private static final Map<Pattern, String> EXPRESSION_REPLACEMENTS = new HashMap<>();
+	
+	// add data to expression replacements
+	static {
+		// add spaces around operators
+		for (final String operator : Arrays.asList("\\*", "/", "\\^")) {
+			EXPRESSION_REPLACEMENTS.put(Pattern.compile(operator),
+					" " + operator + " ");
+		}
+		
+		// replace multiple spaces with a single space
+		EXPRESSION_REPLACEMENTS.put(Pattern.compile(" +"), " ");
+		// place brackets around any expression of the form "number unit", with or
+		// without the space
+		EXPRESSION_REPLACEMENTS.put(Pattern.compile("((?:-?[1-9]\\d*|0)" // integer
+				+ "(?:\\.\\d+(?:[eE]\\d+))?)" // optional decimal point with numbers
+														// after it
+				+ "\\s*" // optional space(s)
+				+ "([a-zA-Z]+(?:\\^\\d+)?" // any string of letters
+				+ "(?:\\s+[a-zA-Z]+(?:\\^\\d+)?))" // optional other letters
+				+ "(?!-?\\d)" // no number directly afterwards (avoids matching
+									// "1e3")
+		), "\\($1 $2\\)");
+	}
+	
+	/**
+	 * A regular expression that separates names and expressions in unit files.
+	 */
+	private static final Pattern NAME_EXPRESSION = Pattern
+			.compile("(\\S+)\\s+(\\S.*)");
+	
+	/**
+	 * The exponent operator
+	 * 
+	 * @param base         base of exponentiation
+	 * @param exponentUnit exponent
+	 * @return result
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private static final LinearUnit exponentiateUnits(final LinearUnit base,
+			final LinearUnit exponentUnit) {
+		// exponent function - first check if o2 is a number,
+		if (exponentUnit.getBase().equals(SI.ONE.getBase())) {
+			// then check if it is an integer,
+			final double exponent = exponentUnit.getConversionFactor();
+			if (DecimalComparison.equals(exponent % 1, 0))
+				// then exponentiate
+				return base.toExponent((int) (exponent + 0.5));
+			else
+				// not an integer
+				throw new UnsupportedOperationException(
+						"Decimal exponents are currently not supported.");
+		} else
+			// not a number
+			throw new IllegalArgumentException("Exponents must be numbers.");
+	}
+	
+	/**
+	 * The exponent operator
+	 * 
+	 * @param base         base of exponentiation
+	 * @param exponentUnit exponent
+	 * @return result
+	 * @since 2020-08-04
+	 */
+	private static final LinearUnitValue exponentiateUnitValues(
+			final LinearUnitValue base, final LinearUnitValue exponentValue) {
+		// exponent function - first check if o2 is a number,
+		if (exponentValue.canConvertTo(SI.ONE)) {
+			// then check if it is an integer,
+			final double exponent = exponentValue.getValueExact();
+			if (DecimalComparison.equals(exponent % 1, 0))
+				// then exponentiate
+				return base.toExponent((int) (exponent + 0.5));
+			else
+				// not an integer
+				throw new UnsupportedOperationException(
+						"Decimal exponents are currently not supported.");
+		} else
+			// not a number
+			throw new IllegalArgumentException("Exponents must be numbers.");
+	}
+	
+	/**
+	 * The units in this system, excluding prefixes.
+	 * 
+	 * @since 2019-01-07
+	 * @since v0.1.0
+	 */
+	private final Map<String, Unit> prefixlessUnits;
+	
+	/**
+	 * The unit prefixes in this system.
+	 * 
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	private final Map<String, UnitPrefix> prefixes;
+	
+	/**
+	 * The dimensions in this system.
+	 * 
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	private final Map<String, ObjectProduct<BaseDimension>> dimensions;
+	
+	/**
+	 * A map mapping strings to units (including prefixes)
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final Map<String, Unit> units;
+	
+	/**
+	 * The rule that specifies when prefix repetition is allowed. It takes in one
+	 * argument: a list of the prefixes being applied to the unit
+	 * <p>
+	 * The prefixes are inputted in <em>application order</em>. This means that
+	 * testing whether "kilomegagigametre" is a valid unit is equivalent to
+	 * running the following code (assuming all variables are defined correctly):
+	 * <br>
+	 * {@code prefixRepetitionRule.test(Arrays.asList(giga, mega, kilo))}
+	 */
+	private Predicate<List<UnitPrefix>> prefixRepetitionRule;
+	
+	/**
+	 * A parser that can parse unit expressions.
+	 * 
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<LinearUnit> unitExpressionParser = new ExpressionParser.Builder<>(
+			this::getLinearUnit).addBinaryOperator("+", (o1, o2) -> o1.plus(o2), 0)
+					.addBinaryOperator("-", (o1, o2) -> o1.minus(o2), 0)
+					.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 1)
+					.addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 1)
+					.addBinaryOperator("^", UnitDatabase::exponentiateUnits, 2)
+					.build();
+	
+	/**
+	 * A parser that can parse unit value expressions.
+	 * 
+	 * @since 2020-08-04
+	 */
+	private final ExpressionParser<LinearUnitValue> unitValueExpressionParser = new ExpressionParser.Builder<>(
+			this::getLinearUnitValue)
+					.addBinaryOperator("+", (o1, o2) -> o1.plus(o2), 0)
+					.addBinaryOperator("-", (o1, o2) -> o1.minus(o2), 0)
+					.addBinaryOperator("*", (o1, o2) -> o1.times(o2), 1)
+					.addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 1)
+					.addBinaryOperator("^", UnitDatabase::exponentiateUnitValues, 2)
+					.build();
+	
+	/**
+	 * A parser that can parse unit prefix expressions
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<UnitPrefix> prefixExpressionParser = new ExpressionParser.Builder<>(
+			this::getPrefix).addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0)
+					.addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0)
+					.addBinaryOperator("^",
+							(o1, o2) -> o1.toExponent(o2.getMultiplier()), 1)
+					.build();
+	
+	/**
+	 * A parser that can parse unit dimension expressions.
+	 * 
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	private final ExpressionParser<ObjectProduct<BaseDimension>> unitDimensionParser = new ExpressionParser.Builder<>(
+			this::getDimension).addBinaryOperator("*", (o1, o2) -> o1.times(o2), 0)
+					.addSpaceFunction("*")
+					.addBinaryOperator("/", (o1, o2) -> o1.dividedBy(o2), 0).build();
+	
+	/**
+	 * Creates the {@code UnitsDatabase}.
+	 * 
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public UnitDatabase() {
+		this(prefixes -> true);
+	}
+	
+	/**
+	 * Creates the {@code UnitsDatabase}
+	 * 
+	 * @param prefixRepetitionRule the rule that determines when prefix
+	 *                             repetition is allowed
+	 * @since 2020-08-26
+	 */
+	public UnitDatabase(Predicate<List<UnitPrefix>> prefixRepetitionRule) {
+		this.prefixlessUnits = new HashMap<>();
+		this.prefixes = new HashMap<>();
+		this.dimensions = new HashMap<>();
+		this.prefixRepetitionRule = prefixRepetitionRule;
+		this.units = ConditionalExistenceCollections.conditionalExistenceMap(
+				new PrefixedUnitMap(this.prefixlessUnits, this.prefixes),
+				entry -> this.prefixRepetitionRule
+						.test(this.getPrefixesFromName(entry.getKey())));
+	}
+	
+	/**
+	 * Adds a unit dimension to the database.
+	 * 
+	 * @param name      dimension's name
+	 * @param dimension dimension to add
+	 * @throws NullPointerException if name or dimension is null
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public void addDimension(final String name,
+			final ObjectProduct<BaseDimension> dimension) {
+		this.dimensions.put(
+				Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(dimension, "dimension must not be null."));
+	}
+	
+	/**
+	 * Adds to the list from a line in a unit dimension file.
+	 * 
+	 * @param line        line to look at
+	 * @param lineCounter number of line, for error messages
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private void addDimensionFromLine(final String line,
+			final long lineCounter) {
+		// ignore lines that start with a # sign - they're comments
+		if (line.isEmpty())
+			return;
+		if (line.contains("#")) {
+			this.addDimensionFromLine(line.substring(0, line.indexOf("#")),
+					lineCounter);
+			return;
+		}
+		
+		// divide line into name and expression
+		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
+		if (!lineMatcher.matches())
+			throw new IllegalArgumentException(String.format(
+					"Error at line %d: Lines of a dimension file must consist of a dimension name, then spaces or tabs, then a dimension expression.",
+					lineCounter));
+		final String name = lineMatcher.group(1);
+		final String expression = lineMatcher.group(2);
+		
+		if (name.endsWith(" ")) {
+			System.err.printf("Warning - line %d's dimension name ends in a space",
+					lineCounter);
+		}
+		
+		// if expression is "!", search for an existing dimension
+		// if no unit found, throw an error
+		if (expression.equals("!")) {
+			if (!this.containsDimensionName(name))
+				throw new IllegalArgumentException(String.format(
+						"! used but no dimension found (line %d).", lineCounter));
+		} else {
+			// it's a unit, get the unit
+			final ObjectProduct<BaseDimension> dimension;
+			try {
+				dimension = this.getDimensionFromExpression(expression);
+			} catch (final IllegalArgumentException e) {
+				System.err.printf("Parsing error on line %d:%n", lineCounter);
+				throw e;
+			}
+			
+			this.addDimension(name, dimension);
+		}
+	}
+	
+	/**
+	 * Adds a unit prefix to the database.
+	 * 
+	 * @param name   prefix's name
+	 * @param prefix prefix to add
+	 * @throws NullPointerException if name or prefix is null
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	public void addPrefix(final String name, final UnitPrefix prefix) {
+		this.prefixes.put(Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(prefix, "prefix must not be null."));
+	}
+	
+	/**
+	 * Adds a unit to the database.
+	 * 
+	 * @param name unit's name
+	 * @param unit unit to add
+	 * @throws NullPointerException if unit is null
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public void addUnit(final String name, final Unit unit) {
+		this.prefixlessUnits.put(
+				Objects.requireNonNull(name, "name must not be null."),
+				Objects.requireNonNull(unit, "unit must not be null."));
+	}
+	
+	/**
+	 * Adds to the list from a line in a unit file.
+	 * 
+	 * @param line        line to look at
+	 * @param lineCounter number of line, for error messages
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	private void addUnitOrPrefixFromLine(final String line,
+			final long lineCounter) {
+		// ignore lines that start with a # sign - they're comments
+		if (line.isEmpty())
+			return;
+		if (line.contains("#")) {
+			this.addUnitOrPrefixFromLine(line.substring(0, line.indexOf("#")),
+					lineCounter);
+			return;
+		}
+		
+		// divide line into name and expression
+		final Matcher lineMatcher = NAME_EXPRESSION.matcher(line);
+		if (!lineMatcher.matches())
+			throw new IllegalArgumentException(String.format(
+					"Error at line %d: Lines of a unit file must consist of a unit name, then spaces or tabs, then a unit expression.",
+					lineCounter));
+		final String name = lineMatcher.group(1);
+		
+		final String expression = lineMatcher.group(2);
+		
+		if (name.endsWith(" ")) {
+			System.err.printf("Warning - line %d's unit name ends in a space",
+					lineCounter);
+		}
+		
+		// if expression is "!", search for an existing unit
+		// if no unit found, throw an error
+		if (expression.equals("!")) {
+			if (!this.containsUnitName(name))
+				throw new IllegalArgumentException(String
+						.format("! used but no unit found (line %d).", lineCounter));
+		} else {
+			if (name.endsWith("-")) {
+				final UnitPrefix prefix;
+				try {
+					prefix = this.getPrefixFromExpression(expression);
+				} catch (final IllegalArgumentException e) {
+					System.err.printf("Parsing error on line %d:%n", lineCounter);
+					throw e;
+				}
+				this.addPrefix(name.substring(0, name.length() - 1), prefix);
+			} else {
+				// it's a unit, get the unit
+				final Unit unit;
+				try {
+					unit = this.getUnitFromExpression(expression);
+				} catch (final IllegalArgumentException e) {
+					System.err.printf("Parsing error on line %d:%n", lineCounter);
+					throw e;
+				}
+				
+				this.addUnit(name, unit);
+			}
+		}
+	}
+	
+	/**
+	 * Tests if the database has a unit dimension with this name.
+	 * 
+	 * @param name name to test
+	 * @return if database contains name
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public boolean containsDimensionName(final String name) {
+		return this.dimensions.containsKey(name);
+	}
+	
+	/**
+	 * Tests if the database has a unit prefix with this name.
+	 * 
+	 * @param name name to test
+	 * @return if database contains name
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public boolean containsPrefixName(final String name) {
+		return this.prefixes.containsKey(name);
+	}
+	
+	/**
+	 * Tests if the database has a unit with this name, taking prefixes into
+	 * consideration
+	 * 
+	 * @param name name to test
+	 * @return if database contains name
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public boolean containsUnitName(final String name) {
+		return this.units.containsKey(name);
+	}
+	
+	/**
+	 * @return a map mapping dimension names to dimensions
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, ObjectProduct<BaseDimension>> dimensionMap() {
+		return Collections.unmodifiableMap(this.dimensions);
+	}
+	
+	/**
+	 * Evaluates a unit expression, following the same rules as
+	 * {@link #getUnitFromExpression}.
+	 * 
+	 * @param expression expression to parse
+	 * @return {@code LinearUnitValue} representing value of expression
+	 * @since 2020-08-04
+	 */
+	public LinearUnitValue evaluateUnitExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		// attempt to get a unit as an alias, or a number with precision first
+		if (this.containsUnitName(expression))
+			return this.getLinearUnitValue(expression);
+		
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		modifiedExpression = modifiedExpression.replaceAll("\\+", " \\+ ");
+		modifiedExpression = modifiedExpression.replaceAll("-", " - ");
+		
+		// format expression
+		for (final Entry<Pattern, String> replacement : EXPRESSION_REPLACEMENTS
+				.entrySet()) {
+			modifiedExpression = replacement.getKey().matcher(modifiedExpression)
+					.replaceAll(replacement.getValue());
+		}
+		
+		// the previous operation breaks negative numbers, fix them!
+		// (i.e. -2 becomes - 2)
+		// FIXME the previous operaton also breaks stuff like "1e-5"
+		for (int i = 0; i < modifiedExpression.length(); i++) {
+			if (modifiedExpression.charAt(i) == '-'
+					&& (i < 2 || Arrays.asList('+', '-', '*', '/', '^')
+							.contains(modifiedExpression.charAt(i - 2)))) {
+				// found a broken negative number
+				modifiedExpression = modifiedExpression.substring(0, i + 1)
+						+ modifiedExpression.substring(i + 2);
+			}
+		}
+		
+		return this.unitValueExpressionParser.parseExpression(modifiedExpression);
+	}
+	
+	/**
+	 * Gets a unit dimension from the database using its name.
+	 * 
+	 * <p>
+	 * This method accepts exponents, like "L^3"
+	 * </p>
+	 * 
+	 * @param name dimension's name
+	 * @return dimension
+	 * @since 2019-03-14
+	 * @since v0.2.0
+	 */
+	public ObjectProduct<BaseDimension> getDimension(final String name) {
+		Objects.requireNonNull(name, "name must not be null.");
+		if (name.contains("^")) {
+			final String[] baseAndExponent = name.split("\\^");
+			
+			final ObjectProduct<BaseDimension> base = this
+					.getDimension(baseAndExponent[0]);
+			
+			final int exponent;
+			try {
+				exponent = Integer
+						.parseInt(baseAndExponent[baseAndExponent.length - 1]);
+			} catch (final NumberFormatException e2) {
+				throw new IllegalArgumentException("Exponent must be an integer.");
+			}
+			
+			return base.toExponent(exponent);
+		}
+		return this.dimensions.get(name);
+	}
+	
+	/**
+	 * Uses the database's data to parse an expression into a unit dimension
+	 * <p>
+	 * The expression is a series of any of the following:
+	 * <ul>
+	 * <li>The name of a unit dimension, which multiplies or divides the result
+	 * based on preceding operators</li>
+	 * <li>The operators '*' and '/', which multiply and divide (note that just
+	 * putting two unit dimensions next to each other is equivalent to
+	 * multiplication)</li>
+	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
+	 * </ul>
+	 * 
+	 * @param expression expression to parse
+	 * @throws IllegalArgumentException if the expression cannot be parsed
+	 * @throws NullPointerException     if expression is null
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public ObjectProduct<BaseDimension> getDimensionFromExpression(
+			final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		// attempt to get a dimension as an alias first
+		if (this.containsDimensionName(expression))
+			return this.getDimension(expression);
+		
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		
+		// format expression
+		for (final Entry<Pattern, String> replacement : EXPRESSION_REPLACEMENTS
+				.entrySet()) {
+			modifiedExpression = replacement.getKey().matcher(modifiedExpression)
+					.replaceAll(replacement.getValue());
+		}
+		modifiedExpression = modifiedExpression.replaceAll(" *\\^ *", "\\^");
+		
+		return this.unitDimensionParser.parseExpression(modifiedExpression);
+	}
+	
+	/**
+	 * Gets a unit. If it is linear, cast it to a LinearUnit and return it.
+	 * Otherwise, throw an {@code IllegalArgumentException}.
+	 * 
+	 * @param name unit's name
+	 * @return unit
+	 * @since 2019-03-22
+	 * @since v0.2.0
+	 */
+	private LinearUnit getLinearUnit(final String name) {
+		// see if I am using a function-unit like tempC(100)
+		Objects.requireNonNull(name, "name may not be null");
+		if (name.contains("(") && name.contains(")")) {
+			// break it into function name and value
+			final List<String> parts = Arrays.asList(name.split("\\("));
+			if (parts.size() != 2)
+				throw new IllegalArgumentException(
+						"Format nonlinear units like: unit(value).");
+			
+			// solve the function
+			final Unit unit = this.getUnit(parts.get(0));
+			final double value = Double.parseDouble(
+					parts.get(1).substring(0, parts.get(1).length() - 1));
+			return LinearUnit.fromUnitValue(unit, value);
+		} else {
+			// get a linear unit
+			final Unit unit = this.getUnit(name);
+			
+			if (unit instanceof LinearUnit)
+				return (LinearUnit) unit;
+			else
+				throw new IllegalArgumentException(
+						String.format("%s is not a linear unit.", name));
+		}
+	}
+	
+	/**
+	 * Gets a {@code LinearUnitValue} from a unit name. Nonlinear units will be
+	 * converted to their base units.
+	 * 
+	 * @param name name of unit
+	 * @return {@code LinearUnitValue} instance
+	 * @since 2020-08-04
+	 */
+	private LinearUnitValue getLinearUnitValue(final String name) {
+		try {
+			// try to parse it as a number - otherwise it is not a number!
+			final BigDecimal number = new BigDecimal(name);
+			
+			final double uncertainty = Math.pow(10, -number.scale());
+			return LinearUnitValue.of(SI.ONE,
+					UncertainDouble.of(number.doubleValue(), uncertainty));
+		} catch (final NumberFormatException e) {
+			return LinearUnitValue.getExact(this.getLinearUnit(name), 1);
+		}
+	}
+	
+	/**
+	 * Gets a unit prefix from the database from its name
+	 * 
+	 * @param name prefix's name
+	 * @return prefix
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public UnitPrefix getPrefix(final String name) {
+		try {
+			return UnitPrefix.valueOf(Double.parseDouble(name));
+		} catch (final NumberFormatException e) {
+			return this.prefixes.get(name);
+		}
+	}
+	
+	/**
+	 * Gets all of the prefixes that are on a unit name, in application order.
+	 * 
+	 * @param unitName name of unit
+	 * @return prefixes
+	 * @since 2020-08-26
+	 */
+	List<UnitPrefix> getPrefixesFromName(final String unitName) {
+		final List<UnitPrefix> prefixes = new ArrayList<>();
+		String name = unitName;
+		
+		while (!this.prefixlessUnits.containsKey(name)) {
+			// find the longest prefix
+			String longestPrefixName = null;
+			int longestLength = name.length();
+			
+			while (longestPrefixName == null) {
+				longestLength--;
+				if (longestLength <= 0)
+					throw new AssertionError(
+							"No prefix found in " + name + ", but it is not a unit!");
+				if (this.prefixes.containsKey(name.substring(0, longestLength))) {
+					longestPrefixName = name.substring(0, longestLength);
+				}
+			}
+			
+			// longest prefix found!
+			final UnitPrefix prefix = this.getPrefix(longestPrefixName);
+			prefixes.add(0, prefix);
+			name = name.substring(longestLength);
+		}
+		return prefixes;
+	}
+	
+	/**
+	 * Gets a unit prefix from a prefix expression
+	 * <p>
+	 * Currently, prefix expressions are much simpler than unit expressions: They
+	 * are either a number or the name of another prefix
+	 * </p>
+	 * 
+	 * @param expression expression to input
+	 * @return prefix
+	 * @throws IllegalArgumentException if expression cannot be parsed
+	 * @throws NullPointerException     if any argument is null
+	 * @since 2019-01-14
+	 * @since v0.1.0
+	 */
+	public UnitPrefix getPrefixFromExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		// attempt to get a unit as an alias first
+		if (this.containsUnitName(expression))
+			return this.getPrefix(expression);
+		
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		
+		// format expression
+		for (final Entry<Pattern, String> replacement : EXPRESSION_REPLACEMENTS
+				.entrySet()) {
+			modifiedExpression = replacement.getKey().matcher(modifiedExpression)
+					.replaceAll(replacement.getValue());
+		}
+		
+		return this.prefixExpressionParser.parseExpression(modifiedExpression);
+	}
+	
+	/**
+	 * @return the prefixRepetitionRule
+	 * @since 2020-08-26
+	 */
+	public final Predicate<List<UnitPrefix>> getPrefixRepetitionRule() {
+		return this.prefixRepetitionRule;
+	}
+	
+	/**
+	 * Gets a unit from the database from its name, looking for prefixes.
+	 * 
+	 * @param name unit's name
+	 * @return unit
+	 * @since 2019-01-10
+	 * @since v0.1.0
+	 */
+	public Unit getUnit(final String name) {
+		try {
+			final double value = Double.parseDouble(name);
+			return SI.ONE.times(value);
+		} catch (final NumberFormatException e) {
+			final Unit unit = this.units.get(name);
+			if (unit == null)
+				throw new NoSuchElementException("No unit " + name);
+			else if (unit.getPrimaryName().isEmpty())
+				return unit.withName(NameSymbol.ofName(name));
+			else if (!unit.getPrimaryName().get().equals(name)) {
+				final Set<String> otherNames = new HashSet<>(unit.getOtherNames());
+				otherNames.add(unit.getPrimaryName().get());
+				return unit.withName(NameSymbol.ofNullable(name,
+						unit.getSymbol().orElse(null), otherNames));
+			} else if (!unit.getOtherNames().contains(name)) {
+				final Set<String> otherNames = new HashSet<>(unit.getOtherNames());
+				otherNames.add(name);
+				return unit.withName(
+						NameSymbol.ofNullable(unit.getPrimaryName().orElse(null),
+								unit.getSymbol().orElse(null), otherNames));
+			} else
+				return unit;
+		}
+		
+	}
+	
+	/**
+	 * Uses the database's unit data to parse an expression into a unit
+	 * <p>
+	 * The expression is a series of any of the following:
+	 * <ul>
+	 * <li>The name of a unit, which multiplies or divides the result based on
+	 * preceding operators</li>
+	 * <li>The operators '*' and '/', which multiply and divide (note that just
+	 * putting two units or values next to each other is equivalent to
+	 * multiplication)</li>
+	 * <li>The operator '^' which exponentiates. Exponents must be integers.</li>
+	 * <li>A number which is multiplied or divided</li>
+	 * </ul>
+	 * This method only works with linear units.
+	 * 
+	 * @param expression expression to parse
+	 * @throws IllegalArgumentException if the expression cannot be parsed
+	 * @throws NullPointerException     if expression is null
+	 * @since 2019-01-07
+	 * @since v0.1.0
+	 */
+	public Unit getUnitFromExpression(final String expression) {
+		Objects.requireNonNull(expression, "expression must not be null.");
+		
+		// attempt to get a unit as an alias first
+		if (this.containsUnitName(expression))
+			return this.getUnit(expression);
+		
+		// force operators to have spaces
+		String modifiedExpression = expression;
+		modifiedExpression = modifiedExpression.replaceAll("\\+", " \\+ ");
+		modifiedExpression = modifiedExpression.replaceAll("-", " - ");
+		
+		// format expression
+		for (final Entry<Pattern, String> replacement : EXPRESSION_REPLACEMENTS
+				.entrySet()) {
+			modifiedExpression = replacement.getKey().matcher(modifiedExpression)
+					.replaceAll(replacement.getValue());
+		}
+		
+		// the previous operation breaks negative numbers, fix them!
+		// (i.e. -2 becomes - 2)
+		for (int i = 0; i < modifiedExpression.length(); i++) {
+			if (modifiedExpression.charAt(i) == '-'
+					&& (i < 2 || Arrays.asList('+', '-', '*', '/', '^')
+							.contains(modifiedExpression.charAt(i - 2)))) {
+				// found a broken negative number
+				modifiedExpression = modifiedExpression.substring(0, i + 1)
+						+ modifiedExpression.substring(i + 2);
+			}
+		}
+		
+		return this.unitExpressionParser.parseExpression(modifiedExpression);
+	}
+	
+	/**
+	 * Adds all dimensions from a file, using data from the database to parse
+	 * them.
+	 * <p>
+	 * Each line in the file should consist of a name and an expression (parsed
+	 * by getDimensionFromExpression) separated by any number of tab characters.
+	 * <p>
+	 * <p>
+	 * Allowed exceptions:
+	 * <ul>
+	 * <li>Anything after a '#' character is considered a comment and
+	 * ignored.</li>
+	 * <li>Blank lines are also ignored</li>
+	 * <li>If an expression consists of a single exclamation point, instead of
+	 * parsing it, this method will search the database for an existing unit. If
+	 * no unit is found, an IllegalArgumentException is thrown. This is used to
+	 * define initial units and ensure that the database contains them.</li>
+	 * </ul>
+	 * 
+	 * @param file file to read
+	 * @throws IllegalArgumentException if the file cannot be parsed, found or
+	 *                                  read
+	 * @throws NullPointerException     if file is null
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public void loadDimensionFile(final Path file) {
+		Objects.requireNonNull(file, "file must not be null.");
+		try {
+			long lineCounter = 0;
+			for (final String line : Files.readAllLines(file)) {
+				this.addDimensionFromLine(line, ++lineCounter);
+			}
+		} catch (final FileNotFoundException e) {
+			throw new IllegalArgumentException("Could not find file " + file, e);
+		} catch (final IOException e) {
+			throw new IllegalArgumentException("Could not read file " + file, e);
+		}
+	}
+	
+	/**
+	 * Adds all dimensions from a {@code InputStream}. Otherwise, works like
+	 * {@link #loadDimensionFile}.
+	 *
+	 * @param stream stream to load from
+	 * @since 2021-03-27
+	 */
+	public void loadDimensionsFromStream(final InputStream stream) {
+		try (final Scanner scanner = new Scanner(stream)) {
+			long lineCounter = 0;
+			while (scanner.hasNextLine()) {
+				this.addDimensionFromLine(scanner.nextLine(), ++lineCounter);
+			}
+		}
+	}
+	
+	/**
+	 * Adds all units from a file, using data from the database to parse them.
+	 * <p>
+	 * Each line in the file should consist of a name and an expression (parsed
+	 * by getUnitFromExpression) separated by any number of tab characters.
+	 * <p>
+	 * <p>
+	 * Allowed exceptions:
+	 * <ul>
+	 * <li>Anything after a '#' character is considered a comment and
+	 * ignored.</li>
+	 * <li>Blank lines are also ignored</li>
+	 * <li>If an expression consists of a single exclamation point, instead of
+	 * parsing it, this method will search the database for an existing unit. If
+	 * no unit is found, an IllegalArgumentException is thrown. This is used to
+	 * define initial units and ensure that the database contains them.</li>
+	 * </ul>
+	 * 
+	 * @param file file to read
+	 * @throws IllegalArgumentException if the file cannot be parsed, found or
+	 *                                  read
+	 * @throws NullPointerException     if file is null
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 */
+	public void loadUnitsFile(final Path file) {
+		Objects.requireNonNull(file, "file must not be null.");
+		try {
+			long lineCounter = 0;
+			for (final String line : Files.readAllLines(file)) {
+				this.addUnitOrPrefixFromLine(line, ++lineCounter);
+			}
+		} catch (final FileNotFoundException e) {
+			throw new IllegalArgumentException("Could not find file " + file, e);
+		} catch (final IOException e) {
+			throw new IllegalArgumentException("Could not read file " + file, e);
+		}
+	}
+	
+	/**
+	 * Adds all units from a {@code InputStream}. Otherwise, works like
+	 * {@link #loadUnitsFile}.
+	 *
+	 * @param stream stream to load from
+	 * @since 2021-03-27
+	 */
+	public void loadUnitsFromStream(InputStream stream) {
+		try (final Scanner scanner = new Scanner(stream)) {
+			long lineCounter = 0;
+			while (scanner.hasNextLine()) {
+				this.addUnitOrPrefixFromLine(scanner.nextLine(), ++lineCounter);
+			}
+		}
+	}
+	
+	/**
+	 * @return a map mapping prefix names to prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, UnitPrefix> prefixMap() {
+		return Collections.unmodifiableMap(this.prefixes);
+	}
+	
+	/**
+	 * @param prefixRepetitionRule the prefixRepetitionRule to set
+	 * @since 2020-08-26
+	 */
+	public final void setPrefixRepetitionRule(
+			Predicate<List<UnitPrefix>> prefixRepetitionRule) {
+		this.prefixRepetitionRule = prefixRepetitionRule;
+	}
+	
+	/**
+	 * @return a string stating the number of units, prefixes and dimensions in
+	 *         the database
+	 */
+	@Override
+	public String toString() {
+		return String.format(
+				"Unit Database with %d units, %d unit prefixes and %d dimensions",
+				this.prefixlessUnits.size(), this.prefixes.size(),
+				this.dimensions.size());
+	}
+	
+	/**
+	 * Returns a map mapping unit names to units, including units with prefixes.
+	 * <p>
+	 * The returned map is infinite in size if there is at least one unit and at
+	 * least one prefix. If it is infinite, some operations that only work with
+	 * finite collections, like converting name/entry sets to arrays, will throw
+	 * an {@code IllegalStateException}.
+	 * </p>
+	 * <p>
+	 * Specifically, the operations that will throw an IllegalStateException if
+	 * the map is infinite in size are:
+	 * <ul>
+	 * <li>{@code unitMap.entrySet().toArray()} (either overloading)</li>
+	 * <li>{@code unitMap.keySet().toArray()} (either overloading)</li>
+	 * </ul>
+	 * </p>
+	 * <p>
+	 * Because of ambiguities between prefixes (i.e. kilokilo = mega), the map's
+	 * {@link PrefixedUnitMap#containsValue containsValue} and
+	 * {@link PrefixedUnitMap#values() values()} methods currently ignore
+	 * prefixes.
+	 * </p>
+	 * 
+	 * @return a map mapping unit names to units, including prefixed names
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, Unit> unitMap() {
+		return this.units; // PrefixedUnitMap is immutable so I don't need to make
+									// an unmodifiable map.
+	}
+	
+	/**
+	 * @return a map mapping unit names to units, ignoring prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public Map<String, Unit> unitMapPrefixless() {
+		return Collections.unmodifiableMap(this.prefixlessUnits);
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/UnitPrefix.java b/src/main/java/org/unitConverter/unit/UnitPrefix.java
new file mode 100644
index 0000000..31cc0b3
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/UnitPrefix.java
@@ -0,0 +1,242 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+import java.util.Optional;
+import java.util.Set;
+
+import org.unitConverter.math.DecimalComparison;
+
+/**
+ * A prefix that can be applied to a {@code LinearUnit} to multiply it by some value
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ */
+public final class UnitPrefix {
+	/**
+	 * Gets a {@code UnitPrefix} from a multiplier
+	 * 
+	 * @param multiplier
+	 *            multiplier of prefix
+	 * @return prefix
+	 * @since 2019-10-16
+	 */
+	public static UnitPrefix valueOf(final double multiplier) {
+		return new UnitPrefix(multiplier, NameSymbol.EMPTY);
+	}
+
+	/**
+	 * Gets a {@code UnitPrefix} from a multiplier and a name
+	 * 
+	 * @param multiplier
+	 *            multiplier of prefix
+	 * @param ns
+	 *            name(s) and symbol of prefix
+	 * @return prefix
+	 * @since 2019-10-16
+	 * @throws NullPointerException
+	 *             if ns is null
+	 */
+	public static UnitPrefix valueOf(final double multiplier, final NameSymbol ns) {
+		return new UnitPrefix(multiplier, Objects.requireNonNull(ns, "ns must not be null."));
+	}
+
+	/**
+	 * This prefix's primary name
+	 */
+	private final Optional<String> primaryName;
+
+	/**
+	 * This prefix's symbol
+	 */
+	private final Optional<String> symbol;
+
+	/**
+	 * Other names and symbols used by this prefix
+	 */
+	private final Set<String> otherNames;
+
+	/**
+	 * The number that this prefix multiplies units by
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final double multiplier;
+
+	/**
+	 * Creates the {@code DefaultUnitPrefix}.
+	 * 
+	 * @param multiplier
+	 * @since 2019-01-14
+	 * @since v0.2.0
+	 */
+	private UnitPrefix(final double multiplier, final NameSymbol ns) {
+		this.multiplier = multiplier;
+		this.primaryName = ns.getPrimaryName();
+		this.symbol = ns.getSymbol();
+		this.otherNames = ns.getOtherNames();
+	}
+
+	/**
+	 * Divides this prefix by a scalar
+	 * 
+	 * @param divisor
+	 *            number to divide by
+	 * @return quotient of prefix and scalar
+	 * @since 2019-10-16
+	 */
+	public UnitPrefix dividedBy(final double divisor) {
+		return valueOf(this.getMultiplier() / divisor);
+	}
+
+	/**
+	 * Divides this prefix by {@code other}.
+	 * 
+	 * @param other
+	 *            prefix to divide by
+	 * @return quotient of prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public UnitPrefix dividedBy(final UnitPrefix other) {
+		return valueOf(this.getMultiplier() / other.getMultiplier());
+	}
+
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses the prefix's multiplier to determine equality.
+	 */
+	@Override
+	public boolean equals(final Object obj) {
+		if (this == obj)
+			return true;
+		if (obj == null)
+			return false;
+		if (!(obj instanceof UnitPrefix))
+			return false;
+		final UnitPrefix other = (UnitPrefix) obj;
+		return DecimalComparison.equals(this.getMultiplier(), other.getMultiplier());
+	}
+
+	/**
+	 * @return prefix's multiplier
+	 * @since 2019-11-26
+	 */
+	public double getMultiplier() {
+		return this.multiplier;
+	}
+
+	/**
+	 * @return other names
+	 * @since 2019-11-26
+	 */
+	public final Set<String> getOtherNames() {
+		return this.otherNames;
+	}
+
+	/**
+	 * @return primary name
+	 * @since 2019-11-26
+	 */
+	public final Optional<String> getPrimaryName() {
+		return this.primaryName;
+	}
+
+	/**
+	 * @return symbol
+	 * @since 2019-11-26
+	 */
+	public final Optional<String> getSymbol() {
+		return this.symbol;
+	}
+
+	/**
+	 * {@inheritDoc}
+	 * 
+	 * Uses the prefix's multiplier to determine a hash code.
+	 */
+	@Override
+	public int hashCode() {
+		return DecimalComparison.hash(this.getMultiplier());
+	}
+
+	/**
+	 * Multiplies this prefix by a scalar
+	 * 
+	 * @param multiplicand
+	 *            number to multiply by
+	 * @return product of prefix and scalar
+	 * @since 2019-10-16
+	 */
+	public UnitPrefix times(final double multiplicand) {
+		return valueOf(this.getMultiplier() * multiplicand);
+	}
+
+	/**
+	 * Multiplies this prefix by {@code other}.
+	 * 
+	 * @param other
+	 *            prefix to multiply by
+	 * @return product of prefixes
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public UnitPrefix times(final UnitPrefix other) {
+		return valueOf(this.getMultiplier() * other.getMultiplier());
+	}
+
+	/**
+	 * Raises this prefix to an exponent.
+	 * 
+	 * @param exponent
+	 *            exponent to raise to
+	 * @return result of exponentiation.
+	 * @since 2019-04-13
+	 * @since v0.2.0
+	 */
+	public UnitPrefix toExponent(final double exponent) {
+		return valueOf(Math.pow(this.getMultiplier(), exponent));
+	}
+
+	/**
+	 * @return a string describing the prefix and its multiplier
+	 */
+	@Override
+	public String toString() {
+		if (this.primaryName.isPresent())
+			return String.format("%s (\u00D7 %s)", this.primaryName.get(), this.multiplier);
+		else if (this.symbol.isPresent())
+			return String.format("%s (\u00D7 %s)", this.symbol.get(), this.multiplier);
+		else
+			return String.format("Unit Prefix (\u00D7 %s)", this.multiplier);
+	}
+
+	/**
+	 * @param ns
+	 *            name(s) and symbol to use
+	 * @return copy of this prefix with provided name(s) and symbol
+	 * @since 2019-11-26
+	 * @throws NullPointerException
+	 *             if ns is null
+	 */
+	public UnitPrefix withName(final NameSymbol ns) {
+		return valueOf(this.multiplier, ns);
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/UnitValue.java b/src/main/java/org/unitConverter/unit/UnitValue.java
new file mode 100644
index 0000000..c138332
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/UnitValue.java
@@ -0,0 +1,172 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Objects;
+import java.util.Optional;
+
+/**
+ * A value expressed in a unit.
+ * 
+ * Unless otherwise indicated, all methods in this class throw a
+ * {@code NullPointerException} when an argument is null.
+ * 
+ * @author Adrien Hopkins
+ * @since 2020-07-26
+ */
+public final class UnitValue {
+	/**
+	 * Creates a {@code UnitValue} from a unit and the associated value.
+	 * 
+	 * @param unit  unit to use
+	 * @param value value to use
+	 * @return {@code UnitValue} instance
+	 */
+	public static UnitValue of(Unit unit, double value) {
+		return new UnitValue(
+				Objects.requireNonNull(unit, "unit must not be null"), value);
+	}
+	
+	private final Unit unit;
+	private final double value;
+	
+	/**
+	 * @param unit  the unit being used
+	 * @param value the value being represented
+	 */
+	private UnitValue(Unit unit, Double value) {
+		this.unit = unit;
+		this.value = value;
+	}
+	
+	/**
+	 * @return true if this value can be converted to {@code other}.
+	 * @since 2020-10-01
+	 */
+	public final boolean canConvertTo(Unit other) {
+		return this.unit.canConvertTo(other);
+	}
+	
+	/**
+	 * @return true if this value can be converted to {@code other}.
+	 * @since 2020-10-01
+	 */
+	public final <W> boolean canConvertTo(Unitlike<W> other) {
+		return this.unit.canConvertTo(other);
+	}
+	
+	/**
+	 * Returns a UnitlikeValue that represents the same value expressed in a
+	 * different unitlike form.
+	 * 
+	 * @param other new unit to express value in
+	 * @return value expressed in {@code other}
+	 */
+	public final <U extends Unitlike<W>, W> UnitlikeValue<U, W> convertTo(
+			U other) {
+		return UnitlikeValue.of(other,
+				this.unit.convertTo(other, this.getValue()));
+	}
+	
+	/**
+	 * Returns a UnitValue that represents the same value expressed in a
+	 * different unit
+	 * 
+	 * @param other new unit to express value in
+	 * @return value expressed in {@code other}
+	 */
+	public final UnitValue convertTo(Unit other) {
+		return UnitValue.of(other,
+				this.getUnit().convertTo(other, this.getValue()));
+	}
+	
+	/**
+	 * Returns this unit value represented as a {@code LinearUnitValue} with this
+	 * unit's base unit as the base.
+	 *
+	 * @param ns name and symbol for the base unit, use NameSymbol.EMPTY if not
+	 *           needed.
+	 * @since 2020-09-29
+	 */
+	public final LinearUnitValue convertToBase(NameSymbol ns) {
+		final LinearUnit base = LinearUnit.getBase(this.unit).withName(ns);
+		return this.convertToLinear(base);
+	}
+	
+	/**
+	 * @return a {@code LinearUnitValue} that is equivalent to this value. It
+	 *         will have zero uncertainty.
+	 * @since 2020-09-29
+	 */
+	public final LinearUnitValue convertToLinear(LinearUnit other) {
+		return LinearUnitValue.getExact(other,
+				this.getUnit().convertTo(other, this.getValue()));
+	}
+	
+	/**
+	 * Returns true if this and obj represent the same value, regardless of
+	 * whether or not they are expressed in the same unit. So (1000 m).equals(1
+	 * km) returns true.
+	 */
+	@Override
+	public boolean equals(Object obj) {
+		if (!(obj instanceof UnitValue))
+			return false;
+		final UnitValue other = (UnitValue) obj;
+		return Objects.equals(this.getUnit().getBase(), other.getUnit().getBase())
+				&& Double.doubleToLongBits(
+						this.getUnit().convertToBase(this.getValue())) == Double
+								.doubleToLongBits(
+										other.getUnit().convertToBase(other.getValue()));
+	}
+	
+	/**
+	 * @return the unit
+	 * @since 2020-09-29
+	 */
+	public final Unit getUnit() {
+		return this.unit;
+	}
+	
+	/**
+	 * @return the value
+	 * @since 2020-09-29
+	 */
+	public final double getValue() {
+		return this.value;
+	}
+	
+	@Override
+	public int hashCode() {
+		return Objects.hash(this.getUnit().getBase(),
+				this.getUnit().convertFromBase(this.getValue()));
+	}
+	
+	@Override
+	public String toString() {
+		final Optional<String> primaryName = this.getUnit().getPrimaryName();
+		final Optional<String> symbol = this.getUnit().getSymbol();
+		if (primaryName.isEmpty() && symbol.isEmpty()) {
+			final double baseValue = this.getUnit().convertToBase(this.getValue());
+			return String.format("%s unnamed unit (= %s %s)", this.getValue(),
+					baseValue, this.getUnit().getBase());
+		} else {
+			final String unitName = symbol.orElse(primaryName.get());
+			return this.getValue() + " " + unitName;
+		}
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/Unitlike.java b/src/main/java/org/unitConverter/unit/Unitlike.java
new file mode 100644
index 0000000..8077771
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/Unitlike.java
@@ -0,0 +1,260 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.HashMap;
+import java.util.Map;
+import java.util.Objects;
+import java.util.function.DoubleFunction;
+import java.util.function.ToDoubleFunction;
+
+import org.unitConverter.math.ObjectProduct;
+
+/**
+ * An object that can convert a value between multiple forms (instances of the
+ * object); like a unit but the "converted value" can be any type.
+ *
+ * @since 2020-09-07
+ */
+public abstract class Unitlike<V> implements Nameable {
+	/**
+	 * Returns a unitlike form from its base and the functions it uses to convert
+	 * to and from its base.
+	 * 
+	 * @param base          unitlike form's base
+	 * @param converterFrom function that accepts a value expressed in the
+	 *                      unitlike form's base and returns that value expressed
+	 *                      in this unitlike form.
+	 * @param converterTo   function that accepts a value expressed in the
+	 *                      unitlike form and returns that value expressed in the
+	 *                      unit's base.
+	 * @return a unitlike form that uses the provided functions to convert.
+	 * @since 2020-09-07
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final <W> Unitlike<W> fromConversionFunctions(
+			final ObjectProduct<BaseUnit> base,
+			final DoubleFunction<W> converterFrom,
+			final ToDoubleFunction<W> converterTo) {
+		return new FunctionalUnitlike<>(base, NameSymbol.EMPTY, converterFrom,
+				converterTo);
+	}
+	
+	/**
+	 * Returns a unitlike form from its base and the functions it uses to convert
+	 * to and from its base.
+	 * 
+	 * @param base          unitlike form's base
+	 * @param converterFrom function that accepts a value expressed in the
+	 *                      unitlike form's base and returns that value expressed
+	 *                      in this unitlike form.
+	 * @param converterTo   function that accepts a value expressed in the
+	 *                      unitlike form and returns that value expressed in the
+	 *                      unit's base.
+	 * @param ns            names and symbol of unit
+	 * @return a unitlike form that uses the provided functions to convert.
+	 * @since 2020-09-07
+	 * @throws NullPointerException if any argument is null
+	 */
+	public static final <W> Unitlike<W> fromConversionFunctions(
+			final ObjectProduct<BaseUnit> base,
+			final DoubleFunction<W> converterFrom,
+			final ToDoubleFunction<W> converterTo, final NameSymbol ns) {
+		return new FunctionalUnitlike<>(base, ns, converterFrom, converterTo);
+	}
+	
+	/**
+	 * The combination of units that this unit is based on.
+	 * 
+	 * @since 2019-10-16
+	 */
+	private final ObjectProduct<BaseUnit> unitBase;
+	
+	/**
+	 * This unit's name(s) and symbol
+	 * 
+	 * @since 2020-09-07
+	 */
+	private final NameSymbol nameSymbol;
+	
+	/**
+	 * Cache storing the result of getDimension()
+	 *
+	 * @since 2019-10-16
+	 */
+	private transient ObjectProduct<BaseDimension> dimension = null;
+	
+	/**
+	 * @param unitBase
+	 * @since 2020-09-07
+	 */
+	protected Unitlike(ObjectProduct<BaseUnit> unitBase, NameSymbol ns) {
+		this.unitBase = Objects.requireNonNull(unitBase,
+				"unitBase may not be null");
+		this.nameSymbol = Objects.requireNonNull(ns, "ns may not be null");
+	}
+	
+	/**
+	 * Checks if a value expressed in this unitlike form can be converted to a
+	 * value expressed in {@code other}
+	 * 
+	 * @param other unit or unitlike form to test with
+	 * @return true if they are compatible
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 * @throws NullPointerException if other is null
+	 */
+	public final boolean canConvertTo(final Unit other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		return Objects.equals(this.getBase(), other.getBase());
+	}
+	
+	/**
+	 * Checks if a value expressed in this unitlike form can be converted to a
+	 * value expressed in {@code other}
+	 * 
+	 * @param other unit or unitlike form to test with
+	 * @return true if they are compatible
+	 * @since 2019-01-13
+	 * @since v0.1.0
+	 * @throws NullPointerException if other is null
+	 */
+	public final <W> boolean canConvertTo(final Unitlike<W> other) {
+		Objects.requireNonNull(other, "other must not be null.");
+		return Objects.equals(this.getBase(), other.getBase());
+	}
+	
+	protected abstract V convertFromBase(double value);
+	
+	/**
+	 * Converts a value expressed in this unitlike form to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 				
+	 * @param other unit to convert to
+	 * @param value value to convert
+	 * @return converted value
+	 * @since 2019-05-22
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unitlike form (as
+	 *                                  tested by {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final double convertTo(final Unit other, final V value) {
+		Objects.requireNonNull(other, "other must not be null.");
+		if (this.canConvertTo(other))
+			return other.convertFromBase(this.convertToBase(value));
+		else
+			throw new IllegalArgumentException(
+					String.format("Cannot convert from %s to %s.", this, other));
+	}
+	
+	/**
+	 * Converts a value expressed in this unitlike form to a value expressed in
+	 * {@code other}.
+	 * 
+	 * @implSpec If conversion is possible, this implementation returns
+	 *           {@code other.convertFromBase(this.convertToBase(value))}.
+	 *           Therefore, overriding either of those methods will change the
+	 *           output of this method.
+	 * 
+	 * @param other unitlike form to convert to
+	 * @param value value to convert
+	 * @param <W>   type of value to convert to
+	 * @return converted value
+	 * @since 2020-09-07
+	 * @throws IllegalArgumentException if {@code other} is incompatible for
+	 *                                  conversion with this unitlike form (as
+	 *                                  tested by {@link Unit#canConvertTo}).
+	 * @throws NullPointerException     if other is null
+	 */
+	public final <W> W convertTo(final Unitlike<W> other, final V value) {
+		Objects.requireNonNull(other, "other must not be null.");
+		if (this.canConvertTo(other))
+			return other.convertFromBase(this.convertToBase(value));
+		else
+			throw new IllegalArgumentException(
+					String.format("Cannot convert from %s to %s.", this, other));
+	}
+	
+	protected abstract double convertToBase(V value);
+	
+	/**
+	 * @return combination of units that this unit is based on
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public final ObjectProduct<BaseUnit> getBase() {
+		return this.unitBase;
+	}
+	
+	/**
+	 * @return dimension measured by this unit
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public final ObjectProduct<BaseDimension> getDimension() {
+		if (this.dimension == null) {
+			final Map<BaseUnit, Integer> mapping = this.unitBase.exponentMap();
+			final Map<BaseDimension, Integer> dimensionMap = new HashMap<>();
+			
+			for (final BaseUnit key : mapping.keySet()) {
+				dimensionMap.put(key.getBaseDimension(), mapping.get(key));
+			}
+			
+			this.dimension = ObjectProduct.fromExponentMapping(dimensionMap);
+		}
+		return this.dimension;
+	}
+	
+	/**
+	 * @return the nameSymbol
+	 * @since 2020-09-07
+	 */
+	@Override
+	public final NameSymbol getNameSymbol() {
+		return this.nameSymbol;
+	}
+	
+	@Override
+	public String toString() {
+		return this.getPrimaryName().orElse("Unnamed unitlike form")
+				+ (this.getSymbol().isPresent()
+						? String.format(" (%s)", this.getSymbol().get())
+						: "")
+				+ ", derived from "
+				+ this.getBase().toString(u -> u.getSymbol().get())
+				+ (this.getOtherNames().isEmpty() ? ""
+						: ", also called " + String.join(", ", this.getOtherNames()));
+	}
+	
+	/**
+	 * @param ns name(s) and symbol to use
+	 * @return a copy of this unitlike form with provided name(s) and symbol
+	 * @since 2020-09-07
+	 * @throws NullPointerException if ns is null
+	 */
+	public Unitlike<V> withName(final NameSymbol ns) {
+		return fromConversionFunctions(this.getBase(), this::convertFromBase,
+				this::convertToBase,
+				Objects.requireNonNull(ns, "ns must not be null."));
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/UnitlikeValue.java b/src/main/java/org/unitConverter/unit/UnitlikeValue.java
new file mode 100644
index 0000000..79201c4
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/UnitlikeValue.java
@@ -0,0 +1,174 @@
+/**
+ * Copyright (C) 2020 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+package org.unitConverter.unit;
+
+import java.util.Optional;
+
+/**
+ *
+ * @since 2020-09-07
+ */
+final class UnitlikeValue<T extends Unitlike<V>, V> {
+	/**
+	 * Gets a {@code UnitlikeValue<V>}.
+	 *
+	 * @since 2020-10-02
+	 */
+	public static <T extends Unitlike<V>, V> UnitlikeValue<T, V> of(T unitlike,
+			V value) {
+		return new UnitlikeValue<>(unitlike, value);
+	}
+	
+	private final T unitlike;
+	private final V value;
+	
+	/**
+	 * @param unitlike
+	 * @param value
+	 * @since 2020-09-07
+	 */
+	private UnitlikeValue(T unitlike, V value) {
+		this.unitlike = unitlike;
+		this.value = value;
+	}
+	
+	/**
+	 * @return true if this value can be converted to {@code other}.
+	 * @since 2020-10-01
+	 */
+	public final boolean canConvertTo(Unit other) {
+		return this.unitlike.canConvertTo(other);
+	}
+	
+	/**
+	 * @return true if this value can be converted to {@code other}.
+	 * @since 2020-10-01
+	 */
+	public final <W> boolean canConvertTo(Unitlike<W> other) {
+		return this.unitlike.canConvertTo(other);
+	}
+	
+	/**
+	 * Returns a UnitlikeValue that represents the same value expressed in a
+	 * different unitlike form.
+	 * 
+	 * @param other new unit to express value in
+	 * @return value expressed in {@code other}
+	 */
+	public final <U extends Unitlike<W>, W> UnitlikeValue<U, W> convertTo(
+			U other) {
+		return UnitlikeValue.of(other,
+				this.unitlike.convertTo(other, this.getValue()));
+	}
+	
+	/**
+	 * Returns a UnitValue that represents the same value expressed in a
+	 * different unit
+	 * 
+	 * @param other new unit to express value in
+	 * @return value expressed in {@code other}
+	 */
+	public final UnitValue convertTo(Unit other) {
+		return UnitValue.of(other,
+				this.unitlike.convertTo(other, this.getValue()));
+	}
+	
+	/**
+	 * Returns this unit value represented as a {@code LinearUnitValue} with this
+	 * unit's base unit as the base.
+	 *
+	 * @param ns name and symbol for the base unit, use NameSymbol.EMPTY if not
+	 *           needed.
+	 * @since 2020-09-29
+	 */
+	public final LinearUnitValue convertToBase(NameSymbol ns) {
+		final LinearUnit base = LinearUnit.getBase(this.unitlike).withName(ns);
+		return this.convertToLinear(base);
+	}
+	
+	/**
+	 * @return a {@code LinearUnitValue} that is equivalent to this value. It
+	 *         will have zero uncertainty.
+	 * @since 2020-09-29
+	 */
+	public final LinearUnitValue convertToLinear(LinearUnit other) {
+		return LinearUnitValue.getExact(other,
+				this.getUnitlike().convertTo(other, this.getValue()));
+	}
+	
+	@Override
+	public boolean equals(Object obj) {
+		if (this == obj)
+			return true;
+		if (!(obj instanceof UnitlikeValue))
+			return false;
+		final UnitlikeValue<?, ?> other = (UnitlikeValue<?, ?>) obj;
+		if (this.getUnitlike() == null) {
+			if (other.getUnitlike() != null)
+				return false;
+		} else if (!this.getUnitlike().equals(other.getUnitlike()))
+			return false;
+		if (this.getValue() == null) {
+			if (other.getValue() != null)
+				return false;
+		} else if (!this.getValue().equals(other.getValue()))
+			return false;
+		return true;
+	}
+	
+	/**
+	 * @return the unitlike
+	 * @since 2020-09-29
+	 */
+	public final Unitlike<V> getUnitlike() {
+		return this.unitlike;
+	}
+	
+	/**
+	 * @return the value
+	 * @since 2020-09-29
+	 */
+	public final V getValue() {
+		return this.value;
+	}
+	
+	@Override
+	public int hashCode() {
+		final int prime = 31;
+		int result = 1;
+		result = prime * result
+				+ (this.getUnitlike() == null ? 0 : this.getUnitlike().hashCode());
+		result = prime * result
+				+ (this.getValue() == null ? 0 : this.getValue().hashCode());
+		return result;
+	}
+	
+	@Override
+	public String toString() {
+		final Optional<String> primaryName = this.getUnitlike().getPrimaryName();
+		final Optional<String> symbol = this.getUnitlike().getSymbol();
+		if (primaryName.isEmpty() && symbol.isEmpty()) {
+			final double baseValue = this.getUnitlike()
+					.convertToBase(this.getValue());
+			return String.format("%s unnamed unit (= %s %s)", this.getValue(),
+					baseValue, this.getUnitlike().getBase());
+		} else {
+			final String unitName = symbol.orElse(primaryName.get());
+			return this.getValue() + " " + unitName;
+		}
+	}
+}
diff --git a/src/main/java/org/unitConverter/unit/package-info.java b/src/main/java/org/unitConverter/unit/package-info.java
new file mode 100644
index 0000000..2f0e097
--- /dev/null
+++ b/src/main/java/org/unitConverter/unit/package-info.java
@@ -0,0 +1,24 @@
+/**
+ * Copyright (C) 2019 Adrien Hopkins
+ *
+ * This program is free software: you can redistribute it and/or modify
+ * it under the terms of the GNU Affero General Public License as published by
+ * the Free Software Foundation, either version 3 of the License, or
+ * (at your option) any later version.
+ *
+ * This program is distributed in the hope that it will be useful,
+ * but WITHOUT ANY WARRANTY; without even the implied warranty of
+ * MERCHANTABILITY or FITNESS FOR A PARTICULAR PURPOSE.  See the
+ * GNU Affero General Public License for more details.
+ *
+ * You should have received a copy of the GNU Affero General Public License
+ * along with this program.  If not, see <https://www.gnu.org/licenses/>.
+ */
+/**
+ * Everything to do with the units that make up Unit Converter.
+ * 
+ * @author Adrien Hopkins
+ * @since 2019-10-16
+ * @since v0.1.0
+ */
+package org.unitConverter.unit;
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