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diff --git a/src/org/unitConverter/unit/BaseUnit.java b/src/org/unitConverter/unit/BaseUnit.java
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+/**
+ * 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.Objects;
+
+import org.unitConverter.dimension.StandardDimensions;
+import org.unitConverter.dimension.UnitDimension;
+
+/**
+ * A unit that is the base for its dimension. It does not have to be for a base dimension, so units like the Newton and
+ * Joule are still base units.
+ * <p>
+ * {@code BaseUnit} does not have any public constructors or static factories. There are two ways to obtain
+ * {@code BaseUnit} instances.
+ * <ol>
+ * <li>The class {@link SI} in this package has constants for all of the SI base units. You can use these constants and
+ * multiply or divide them to get other units. For example:
+ * 
+ * <pre>
+ * BaseUnit JOULE = SI.KILOGRAM.times(SI.METRE.toExponent(2)).dividedBy(SI.SECOND.toExponent(2));
+ * </pre>
+ * 
+ * </li>
+ * <li>You can also query a unit system for a base unit using a unit dimension. The previously mentioned {@link SI}
+ * class can do this for SI and SI-derived units (including imperial and USC), but if you want to use another system,
+ * this is the way to do it. {@link StandardDimensions} contains common unit dimensions that you can use for this. Here
+ * is an example:
+ * 
+ * <pre>
+ * BaseUnit JOULE = SI.SI.getBaseUnit(StandardDimensions.ENERGY);
+ * </pre>
+ * 
+ * </li>
+ * </ol>
+ * 
+ * @author Adrien Hopkins
+ * @since 2018-12-23
+ * @since v0.1.0
+ */
+public final class BaseUnit extends LinearUnit {
+	/**
+	 * Is this unit a full base (i.e. m, s, ... but not N, J, ...)
+	 * 
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	private final boolean isFullBase;
+
+	/**
+	 * Creates the {@code BaseUnit}.
+	 * 
+	 * @param dimension
+	 *            dimension measured by unit
+	 * @param system
+	 *            system that unit is a part of
+	 * @param name
+	 *            name of unit
+	 * @param symbol
+	 *            symbol of unit
+	 * @since 2018-12-23
+	 * @since v0.1.0
+	 */
+	BaseUnit(final UnitDimension dimension, final UnitSystem system) {
+		super(dimension, system, 1);
+		this.isFullBase = dimension.isBase();
+	}
+
+	/**
+	 * Returns the quotient of this unit and another.
+	 * <p>
+	 * Two units can be divided if they are part of the same unit system. If {@code divisor} does not meet this
+	 * condition, an {@code IllegalArgumentException} should be thrown.
+	 * </p>
+	 * 
+	 * @param divisor
+	 *            unit to divide by
+	 * @return quotient of two units
+	 * @throws IllegalArgumentException
+	 *             if {@code divisor} is not compatible for division as described above
+	 * @throws NullPointerException
+	 *             if {@code divisor} is null
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public BaseUnit dividedBy(final BaseUnit divisor) {
+		Objects.requireNonNull(divisor, "other must not be null.");
+
+		// check that these units can be multiplied
+		if (!this.getSystem().equals(divisor.getSystem()))
+			throw new IllegalArgumentException(
+					String.format("Incompatible units for division \"%s\" and \"%s\".", this, divisor));
+
+		return new BaseUnit(this.getDimension().dividedBy(divisor.getDimension()), this.getSystem());
+	}
+
+	/**
+	 * @return true if the unit is a "full base" unit like the metre or second.
+	 * @since 2019-04-10
+	 * @since v0.2.0
+	 */
+	public final boolean isFullBase() {
+		return this.isFullBase;
+	}
+
+	/**
+	 * Returns the product of this unit and another.
+	 * <p>
+	 * Two units can be multiplied if they are part of the same unit system. If {@code multiplier} does not meet this
+	 * condition, an {@code IllegalArgumentException} should be thrown.
+	 * </p>
+	 * 
+	 * @param multiplier
+	 *            unit to multiply by
+	 * @return product of two units
+	 * @throws IllegalArgumentException
+	 *             if {@code multiplier} is not compatible for multiplication as described above
+	 * @throws NullPointerException
+	 *             if {@code multiplier} is null
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public BaseUnit times(final BaseUnit multiplier) {
+		Objects.requireNonNull(multiplier, "other must not be null");
+
+		// check that these units can be multiplied
+		if (!this.getSystem().equals(multiplier.getSystem()))
+			throw new IllegalArgumentException(
+					String.format("Incompatible units for multiplication \"%s\" and \"%s\".", this, multiplier));
+
+		// multiply the units
+		return new BaseUnit(this.getDimension().times(multiplier.getDimension()), this.getSystem());
+	}
+
+	/**
+	 * Returns this unit, but to an exponent.
+	 * 
+	 * @param exponent
+	 *            exponent
+	 * @return result of exponentiation
+	 * @since 2019-01-15
+	 * @since v0.1.0
+	 */
+	@Override
+	public BaseUnit toExponent(final int exponent) {
+		return this.getSystem().getBaseUnit(this.getDimension().toExponent(exponent));
+	}
+
+	@Override
+	public String toString() {
+		return String.format("%s base unit of%s dimension %s", this.getSystem(), this.isFullBase ? " base" : "",
+				this.getDimension());
+	}
+}