For example, the fact that give must occur as the leftmost constituent of the V-bar containing it follows from two conditions. The first is a putatively universal linearisation (i.e. word-order) principle proposed by Stowell (1981, p. 68) which we might call the PERIPHERY PRINCIPLE: this can be outlined informally as in (33) below: (33) PERIPHERY PRINCIPLE (33) The head term of a Phrase appears at the periphery of X-bar What (33) says is that the Head must be the leftmost or rightmost immediate constituent of X-bar.
Source: wiktionary