In linguistics, the head is the word that determines the syntactic type of the phrase of which it is a member, or analogously the stem that determines the semantic category of a compound of which it is a part. The other elements modify the head and are therefore the head's dependents. Headed phrases and compounds are endocentric, whereas exocentric phrases and compounds (if they exist) lack a clear head.
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Examine the following expressions:
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 bird. The birdsong is a kind of song, not a kind of bird. The heads of phrases like the ones here can often be identified by way of constituency tests. For instance, substituting a single word in for 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 tree structures tend to be organized in terms of one of two competing 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, the noun projects its category status up to the mother node, so that the entire phrase is identified as a noun phrase (NP). In the dependency trees, the noun only projects a single node, whereby this node dominates the one node that the adjective projects, a situation that also identifies the entirety as a noun phrase.
The b-trees are structurally the same as their a-counterparts, the only difference being that a different convention is used for marking heads and dependents. The conventions illustrated with trees trees are are just a couple of the various tools that grammarians employ to identify heads and dependents. While other conventions abound, they are usually similar to the ones illustrated here.
Some language typologists classify language syntax according to a head directionality parameter in word order, that is, whether a phrase is head-initial or head-final, assuming that it has a fixed word order at all. English is predominantly a head-initial language, as illustrated with the following dependency tree of the first sentence of Franz Kafka's The Metamorphosis:
The tree shows the extent to which English is primarily a head-initial language. Structure is descending as speech and processing move from left to right. Most dependencies have the head preceding its dependent(s), although there are some head-final dependencies in the tree. For instance, the determiner-noun and adjective-noun dependencies are head-final as well as the subject-verb dependencies. Most other dependencies in English are, however, head-initial as the tree shows. The mixed nature of head-initial and head-final structures is common across languages. In fact purely head-initial or purely head-final languages probably do not exist, although there are some languages that approach purity in this respect, for instance Japanese.
The following tree is of the same sentence from Kafka's novel. The glossing conventions are those established by Lehmann. One can easily see the extent to which Japanese is head-final:
A large majority of head-dependent orderings in Japanese are head-final. This fact is obvious in this tree, since structure is strongly ascending as speech and processing move from left to right. Thus the word order of Japanese is in a sense the opposite of English. One can imagine the difficulties that this state of affairs presents to English speakers learning Japanese and to Japanese speakers learning English.
It is also common to classify language morphology according to whether a phrase is head-marking or dependent-marking. A given dependency is head-marking, if something about the dependent influences the form of the head, and a given dependency is dependent-marking, if something about the head influences the form of the dependent. This distinction is important for languages that have rich systems of inflectional morphology. For English however, the distinction is less central, since English largely lacks inflectional morphology. There are nevertheless a couple of clear cases in English that illustrate the distinction. Examine the following sentences:
The subject-verb dependency in English is head-marking. The third person singular dependent Fred marks its head help-s; it requires the appearance of the 3rd person singular inflection -s. If this -s does not appear, the sentence is bad. Similarly, the 3rd person plural dependent They blocks any ending from appearing on the verb help; if the -s nevertheless appears, the sentence is bad. Compare these examples with the following ones:
Unlike the subject-verb dependency, the determiner-noun dependency in English can be dependent-marking. The singular head donut requires that the demonstrative determiner appear as this/that, and the plural noun donuts requires that the demonstrative determiner take on a plural form these/those. Thus the determiner-noun dependency is dependent-marking in English when the demonstrative this/these/that/those is involved.
The head-marking vs. dependent-marking distinction is comprehensively explored by Johanna Nichols in her seminal article (1986) on head-marking and dependent-marking languages.
In a prosodic unit, the head is that part which extends from the first stressed syllable up to (but not including) the tonic syllable. A high head is the stressed syllable which begins the head and is high in pitch, usually higher than the beginning pitch of the tone on the tonic syllable. For example:
The ↑bus was late.
A low head is the syllable which begins the head and is low in pitch, usually lower than the beginning pitch of the tone on the tonic syllable.
The ↓bus was late.