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In linguistics, the head directionality is a proposed parameter that classifies languages according to whether they are head-initial (the head of a phrase precedes its complements) or head-final (the head follows its complements). (The head is the element that determines the category of a phrase: for example, in a verb phrase, the head is a verb.) Some languages are consistently head-initial or head-final at all phrasal levels. English is considered to be strongly head-initial, while Japanese is an example of a language that is consistently head-final. In certain other languages, such as German and Gbe, examples of both types of head direction occur. Various theories have been proposed to explain such variation. Head directionality is connected with the type of branching that predominates in a language: head-initial structures are ''right-branching'', while head-final structures are ''left-branching''. == Types of phrase == There are various types of phrase in which the ordering of head and complement(s) may be considered when attempting to determine the head directionality of a language, including: #Verb phrase (VP). Here the head is a verb, and the complement(s) are most commonly objects of various types. The ordering here is related to one of the chief questions in the word order typology of languages, namely the normal order of subject, verb and object within a clause (languages are classed on this basis as SVO, SOV, VSO, etc.). #Noun phrase (NP). Here the head is a noun; various kinds of complementizer phrase and adpositional phrase may be considered to be complements. #Adjective phrase (AP). This contains an adjective as the head, and can take as a complement, for example, an adverbial phrase or adpositional phrase. # Adpositional phrase (PP). Such phrases are called prepositional phrases if they are head-initial (i.e. headed by a preposition), or postpositional phrases if they are head-final (i.e. headed by a postposition). For more on these, see Preposition and postposition. The complement is a determiner phrase (or noun phrase, depending on analytical scheme followed). # Determiner phrase (DP). This has a determiner as the head of the phrase. DPs were proposed under generative syntax; not all theories of syntax agree that they exist. #Complementizer phrase (CP). This contains a complementizer, like ''that'' in English, as the head. In some cases the head is covert (not overtly present). The complement can be considered to be a tense phrase. #Tense phrase (TP) and aspect phrase (AspP). These are phrases in which the head is an abstract category representing tense or aspect; the complement is a verb phrase. In more traditional analysis the entire phrase (including any elements denoting tense or aspect) is considered to be simply a verb phrase. In some cases, particularly with noun and adjective phrases, it is not always clear which dependents are to be classed as complements, and which as adjuncts. Although in principle the head-directionality parameter concerns the order of heads and complements only, considerations of head-initiality and head-finality sometimes take account of the position of the head in the phrase as a whole, including adjuncts. The structure of the various types of phrase is analyzed below in relation to specific languages, with a focus on the ordering of head and complement. In some cases (such as English and Japanese) this ordering is found to be the same in practically all types of phrase, whereas in others (such as German and Gbe) the pattern is less consistent. Different theoretical explanations of these inconsistencies are discussed later in the article. 抄文引用元・出典: フリー百科事典『 ウィキペディア（Wikipedia）』 ■ウィキペディアで「Head-directionality parameter」の詳細全文を読む スポンサード リンク
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