An adpositional phrase is a syntactic category that includes prepositional phrases, postpositional phrases, and circumpositional phrases. Adpositional phrases contain an adposition (preposition, postposition, or circumposition) as head and usually a complement such as a noun phrase. Language syntax treats adpositional phrases as units that act as arguments or adjuncts. Prepositional and postpositional phrases differ by the order of the words used. Languages that are primarily head-initial such as English predominantly use prepositional phrases whereas head-final languages predominantly employ postpositional phrases. Many languages have both types, as well as circumpositional phrases.
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Prepositional phrase in the context of Syntactic category
A syntactic category is a syntactic unit that theories of syntax assume. Word classes, largely corresponding to traditional parts of speech (e.g. noun, verb, preposition, etc.), are syntactic categories. In phrase structure grammars, the phrasal categories (e.g. noun phrase, verb phrase, prepositional phrase, etc.) are also syntactic categories. Dependency grammars, however, do not acknowledge phrasal categories (at least not in the traditional sense).
Word classes considered as syntactic categories may be called lexical categories, as distinct from phrasal categories. The terminology is somewhat inconsistent between the theoretical models of different linguists. However, many grammars also draw a distinction between lexical categories (which tend to consist of content words, or phrases headed by them) and functional categories (which tend to consist of function words or abstract functional elements, or phrases headed by them). The term lexical category therefore has two distinct meanings. Moreover, syntactic categories should not be confused with grammatical categories (also known as grammatical features), which are properties such as tense, gender, etc.
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