diff --git a/src/main/java/org/tendiwa/inflectible/GrammaticalCategory.java b/src/main/java/org/tendiwa/inflectible/GrammaticalCategory.java
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+/**
+ * The MIT License (MIT)
+ *
+ * Copyright (c) 2015 Georgy Vlasov
+ *
+ * Permission is hereby granted, free of charge, to any person obtaining a copy
+ * of this software and associated documentation files (the "Software"), to deal
+ * in the Software without restriction, including without limitation the rights
+ * to use, copy, modify, merge, publish, distribute, sublicense, and/or sell
+ * copies of the Software, and to permit persons to whom the Software is
+ * furnished to do so, subject to the following conditions:
+ *
+ * The above copyright notice and this permission notice shall be included in
+ * all copies or substantial portions of the Software.
+ *
+ * THE SOFTWARE IS PROVIDED "AS IS", WITHOUT WARRANTY OF ANY KIND, EXPRESS OR
+ * IMPLIED, INCLUDING BUT NOT LIMITED TO THE WARRANTIES OF MERCHANTABILITY,
+ * FITNESS FOR A PARTICULAR PURPOSE AND NONINFRINGEMENT. IN NO EVENT SHALL THE
+ * AUTHORS OR COPYRIGHT HOLDERS BE LIABLE FOR ANY CLAIM, DAMAGES OR OTHER
+ * LIABILITY, WHETHER IN AN ACTION OF CONTRACT, TORT OR OTHERWISE, ARISING FROM,
+ * OUT OF OR IN CONNECTION WITH THE SOFTWARE OR THE USE OR OTHER DEALINGS IN
+ * THE SOFTWARE.
+ */
+package org.tendiwa.inflectible;
+
+/**
+ * A set of mutually exclusive grammemes.
+ * <p>For example, in English, the grammatical category of Number consists of
+ * Singular and Plural grammemes.
+ * A word form can't be both singular and plural at the same time.
+ * @see <a href="https://en.wikipedia.org/wiki/Grammatical_category>Grammatical
+ *  category article on Wikipedia</a>
+ * @author Georgy Vlasov (suseika@tendiwa.org)
+ * @version $Id$
+ * @since 0.2.0
+ */
+public interface GrammaticalCategory {
+    /**
+     * Default grammeme to be used when it is not specified which grammeme to
+     * use for this grammatical category. Default grammemes are the ones that
+     * are used in headwords in dictionaries. For example, the default
+     * grammeme for the English grammatical category of number is Singular.
+     * @return Default grammeme
+     */
+    Grammeme defaultGrammeme();
+
+    /**
+     * Checks if this grammatical category contains a particular grammeme.
+     * @param grammeme Grammeme
+     * @return True iff this grammatical category contains a particular grammeme
+     */
+    boolean containsGrammeme(Grammeme grammeme);
+}