Principles of X-Bar Theory
All phrases (XPs) are constructed according to the same principles:
I. They are projections of a head of some syntactic category (V, N, A, P, Adv, D, INFL, COMP), and so have an endocentric structure, in which the head is of the same category as the phrase.
II. They have three levels of projection:
(a) a head (symbolized as X or X°):
A head is the lowest (or zero) level of projection, which lends its features to the whole XP. These features are lexical (N, V) and/or functional (e.g. person, number, gender).
Some heads take complements, i.e. obligatory XPs which are sister to the head.
(b) a recursive intermediate level (symbolized as X'):
If there is only one X', it may be sister to the specifier (SPEC) of XP. A specifier is a YP immediately dominated by XP.
If more than one X' is generated, in order to create (optional) adjunct positions, the top X' will be sister to the specifier, and the lower one(s) will be sister to an adjunct.
(c) a phrasal or maximal level (maximal projection; symbolized as Xmax, XP, or X")
The X-Bar model: