Thus the descriptive power of the base component is to be regarded as an important distinction in language use. Nevertheless, the appearance of parasitic gaps in domains relatively inaccessible to ordinary extraction does not affect the structure of the traditional practice of grammarians. On our assumptions, a subset of English sentences interesting on quite independent grounds suffices to account for a descriptive fact. So far, the natural general principle that will subsume this case is, apparently, determined by an abstract underlying order. To provide a constituent structure for T(Z,K), the theory of syntactic features developed earlier is not quite equivalent to a stipulation to place the constructions into these various categories.
see also: WikiPedia -- Chomskybot