In linguistics, the head or nucleus of a phrase is the word that determines the syntactic category of that phrase. For example, the head of the noun phrase boiling hot water is the noun (head noun) water. Analogously, the head of a compound is the stem that determines the semantic category of that compound. For example, the head of the compound noun handbag is bag, since a handbag is a bag, not a hand. The other elements of the phrase or compound modify the head, and are therefore the head's dependents. Headed phrases and compounds are called endocentric, whereas exocentric ("headless") phrases and compounds (if they exist) lack a clear head. Heads are crucial to establishing the direction of branching. Head-initial phrases are right-branching, head-final phrases are left-branching, and head-medial phrases combine left- and right-branching. Basic examples Examine the following expressions: big red dog birdsong The word dog is the head of big red dog since it determines that the phrase is a noun phrase, not an adjective phrase. Because the adjectives big and red modify this head noun, they are its dependents. Similarly, in the compound noun birdsong, the stem song is the head since it determines the basic meaning of the compound. The stem bird modifies this meaning and is therefore dependent on song. Birdsong is a kind of song, not a kind of bird. Conversely, a songbird is a type of bird since the stem bird is the head in this compound. The heads of phrases can often be identified by way of constituency tests. For instance, substituting a single word in place of the phrase big red dog requires the substitute to be a noun (or pronoun), not an adjective. Representing heads Trees Many theories of syntax represent heads by means of tree structures. These trees tend to be organized in terms of one of two relations: either in terms of the constituency relation of phrase structure grammars or the dependency relation of dependency grammars. Both relations are illustrated with the following trees: The constituency relation is shown on the left and the dependency relation on the right. The a-trees identify heads by way of category labels, whereas the b-trees use the words themselves as the labels. The noun stories (N) is the head over the adjective funny (A). In the constituency trees on the left, the noun projects its category status up to the mother