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.