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diff --git a/src/org/unitConverter/unit/AbstractUnit.java b/src/org/unitConverter/unit/AbstractUnit.java
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+++ b/src/org/unitConverter/unit/AbstractUnit.java
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+/**
+ * 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.dimension.UnitDimension;
+
+/**
+ * The default abstract implementation of the {@code Unit} interface.
+ * 
+ * @author Adrien Hopkins
+ * @since 2018-12-22
+ * @since v0.1.0
+ */
+public abstract class AbstractUnit implements Unit {
+	/**
+	 * The dimension, or what the unit measures.
+	 * 
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	private final UnitDimension dimension;
+
+	/**
+	 * The unit's base unit. Values converted by {@code convertFromBase} and {@code convertToBase} are expressed in this
+	 * unit.
+	 * 
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	private final BaseUnit base;
+
+	/**
+	 * The system that this unit is a part of.
+	 * 
+	 * @since 2018-12-23
+	 * @since v0.1.0
+	 */
+	private final UnitSystem system;
+
+	/**
+	 * Creates the {@code AbstractUnit}.
+	 * 
+	 * @param base
+	 *            unit's base
+	 * @throws NullPointerException
+	 *             if name, symbol or base is null
+	 * @since 2018-12-22
+	 * @since v0.1.0
+	 */
+	public AbstractUnit(final BaseUnit base) {
+		this.base = Objects.requireNonNull(base, "base must not be null.");
+		this.dimension = this.base.getDimension();
+		this.system = this.base.getSystem();
+	}
+
+	/**
+	 * Creates the {@code AbstractUnit} using a unique dimension. This constructor is for making base units and should
+	 * only be used by {@code BaseUnit}.
+	 * 
+	 * @param dimension
+	 *            dimension measured by unit
+	 * @param system
+	 *            system that unit is a part of
+	 * @throws AssertionError
+	 *             if this constructor is not run by {@code BaseUnit} or a subclass
+	 * @throws NullPointerException
+	 *             if name, symbol or dimension is null
+	 * @since 2018-12-23
+	 * @since v0.1.0
+	 */
+	AbstractUnit(final UnitDimension dimension, final UnitSystem system) {
+		// try to set this as a base unit
+		if (this instanceof BaseUnit) {
+			this.base = (BaseUnit) this;
+		} else
+			throw new AssertionError();
+
+		this.dimension = Objects.requireNonNull(dimension, "dimension must not be null.");
+		this.system = Objects.requireNonNull(system, "system must not be null.");
+	}
+
+	@Override
+	public final BaseUnit getBase() {
+		return this.base;
+	}
+
+	@Override
+	public final UnitDimension getDimension() {
+		return this.dimension;
+	}
+
+	@Override
+	public final UnitSystem getSystem() {
+		return this.system;
+	}
+
+	@Override
+	public String toString() {
+		return String.format("%s-derived unit of dimension %s", this.getSystem(), this.getDimension());
+	}
+}