 Let's look at the sentence, Mary will never read the books at home to find out how whole sentences are represented. Let's perform a rough constituted analysis first. Mary is a noun phrase, Will is an auxiliary verb and never read the books at home is a verb phrase and together they form a sentence which in the past was analyzed in terms of a flat structure like this. However this analysis was shown to be inadequate for two reasons. First the S constituent would be the only one that allowed for more than two branches and secondly there are arguments suggesting that the auxiliary verb and the verb phrase have to be treated as a constituent independent from the subject noun phrase. Take for example the sentence fragment test. Who will never read the books at home? Mary or the coordination test. Mary will never read the books at home and has always left them in the library. Furthermore auxiliaries appear to be heads of the whole sentence. They determine the properties of the verb and hence the type of verb phrase suggesting that the verb phrase is a sister of the auxiliary. So we have to revise the flat structure. First we replace auxiliary by infill which stands for inflection whose mother is now infill bar or in short I bar and then we adjoin the verb phrase to I bar. The whole sentence then is the inflectional phrase with the subject NP functioning as its specifier. Thus an inflectional phrase is essentially the same as a sentence but it now strictly follows the X bar scheme having a head, the inflection, a compliment, the verb phrase and a specifier like all other constituents.