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diff --git a/src/org/unitConverter/unit/SI.java b/src/org/unitConverter/unit/SI.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.HashSet;
+import java.util.Objects;
+import java.util.Set;
+
+import org.unitConverter.dimension.StandardDimensions;
+import org.unitConverter.dimension.UnitDimension;
+
+/**
+ * The SI, which holds all SI units
+ * 
+ * @author Adrien Hopkins
+ * @since 2018-12-23
+ * @since v0.1.0
+ */
+public enum SI implements UnitSystem {
+	SI;
+
+	/**
+	 * This system's base units.
+	 * 
+	 * @since 2019-01-25
+	 * @since v0.1.0
+	 */
+	private static final Set<BaseUnit> baseUnits = new HashSet<>();
+
+	// base units
+	public static final BaseUnit METRE = SI.getBaseUnit(StandardDimensions.LENGTH);
+	public static final BaseUnit KILOGRAM = SI.getBaseUnit(StandardDimensions.MASS);
+	public static final BaseUnit SECOND = SI.getBaseUnit(StandardDimensions.TIME);
+	public static final BaseUnit AMPERE = SI.getBaseUnit(StandardDimensions.ELECTRIC_CURRENT);
+	public static final BaseUnit KELVIN = SI.getBaseUnit(StandardDimensions.TEMPERATURE);
+	public static final BaseUnit MOLE = SI.getBaseUnit(StandardDimensions.QUANTITY);
+	public static final BaseUnit CANDELA = SI.getBaseUnit(StandardDimensions.LUMINOUS_INTENSITY);
+
+	@Override
+	public BaseUnit getBaseUnit(final UnitDimension dimension) {
+		// try to find an existing unit before creating a new one
+
+		Objects.requireNonNull(dimension, "dimension must not be null.");
+		for (final BaseUnit unit : baseUnits) {
+			// it will be equal since the conditions for equality are dimension and system,
+			// and system is always SI.
+			if (unit.getDimension().equals(dimension))
+				return unit;
+		}
+		// could not find an existing base unit
+		final BaseUnit unit = new BaseUnit(dimension, this);
+		baseUnits.add(unit);
+		return unit;
+	}
+
+	@Override
+	public String getName() {
+		return "SI";
+	}
+}