Phrase structure is the internal structure of a phrase and its constituents. Phrase structure is represented in one of the following formats: by means of a tree structure or by means of labeled brackets.
sentence (i) might have the phrase structure in (ii):
(i) Bill will meet his elder brother at the bus station (ii) IP / | / | NP I' | | \ N | \ | INFL Bill | \ will \ VP / |------\ V | \ | PP meet | | \ NP P NP / | \ | |\ Det AP N at | \ | | | Det N his A brother | | | the bus station elder
The structure in (ii) shows that the string his elder brother is a syntactic constituent which is a Noun Phrase (NP) whose head is the Noun (N) brother. This head is preceded by an Adjectival Phrase (AP) elder, whose head is the adjective (A) elder. The constituent at the bus station is a Prepositional Phrase (PP); the head of this PP is the preposition (P) at which is followed by the NP the bus station. The constituent meet his elder brother at the bus station is a Verbal Phrase or VP, which is headed by the verb (V) meet, followed by two constituents: the NP his elder brother and the PP at the bus station. The whole sentence is construed as an Inflectional Phrase or IP, the head of which is INFLection), here filled with the auxiliary will. INFL is followed by one constituent, the VP meet his elder brother at the bus station, and preceded by one constituent, the NP Bill. The tree structure in (ii) is equivalent to the labelled bracketing in (iii).
(iii) [IP [NP [N Bill] ] [I'[INFL will] [VP [V meet] [NP[Det his] [AP[A elder]] [N brother]] [PP[P at] [NP[Det the] [N bus station]]]]]]
See also X-bar theory.