In the three level theory (X-bar) it is assumed that XP dominates a specifier and X-bar. X-bar, in turn, dominates the head of the projection (XP - X-bar - X) X and one or more complements of X. The ...

Note that the complement rule (4) is an obligatory rule. The PP complement is optional. In a somewhat more minimal theory of x-bar, there is no intermediate node. The is the phrasal level and the head ...