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.
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.
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.
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Dependency grammar (DG) is a class of modern grammatical theories that are all based on the dependency relation (as opposed to the constituency relation of phrase structure) and that can be traced back primarily to the work of Lucien Tesnière. Dependency is the notion that linguistic units, e.g. words, are connected to each other by directed links. The (finite) verb is taken to be the structural center of clause structure. All other syntactic units (words) are either directly or indirectly connected to the verb in terms of the directed links, which are called dependencies.
A dependent-marking language has grammatical markers of agreement and case government between the words of phrases that tend to appear more on dependents than on heads. The distinction between head-marking and dependent-marking was first explored by Johanna Nichols in 1986, and has since become a central criterion in language typology in which languages are classified according to whether they are more head-marking or dependent-marking. Many languages employ both head and dependent-marking, but some employ double-marking, and yet others employ zero-marking.
A noun phrase, or nominal (phrase), is a phrase that has a noun or pronoun as its head or performs the same grammatical function as a noun. Noun phrases are very common cross-linguistically, and they may be the most frequently occurring phrase type. Noun phrases often function as verb subjects and objects, as predicative expressions and as the complements of prepositions. Noun phrases can be embedded inside each other; for instance, the noun phrase some of his constituents contains the shorter noun phrase his constituents.
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