Modern syntactic theory generally proposes that the words in utterances are organised hierarchically into phrases. A commonly adopted structure is that proposed by Chomsky's X-bar theory, in which a phrase has a head, a specifier and a complement. For example, the Prepositional Phrase (PP) right behind the door has the preposition behind as its head, right as the specifier and the door as the complement.
The noun phrase (NP) the door would have a similar structure, e.g. with door at its head and the as its specifier.