Consider structural configuration of the form (5), where α is an immediate constituent of γ: (5) (i) [γ...α...β...] (ii) [γ...β...α...]. Consider the special case of (5) in which γ = ᾱ and β too is an immediate constituent of γ. Thus α is the head of γ and β is one of its complements [...] This configuration is familiar, of course, from the theory of subcategorization. If α is a 0-level category of X-bar theory then the category β must satisfy the subcategorization frame of α. In this case we will say that α "subcategorizes the position occupied by" β.
- Chomsky (1993), a pag.37