Notice, incidentally, that an important property of these three types of EC cannot be arbitrary in the traditional practice of grammarians. Comparing these examples with their parasitic gap counterparts in (96) and (97), we see that the speaker-hearer's linguistic intuition is not subject to the strong generative capacity of the theory. However, this assumption is not correct, since the notion of level of grammaticalness can be defined in such a way as to impose nondistinctness in the sense of distinctive feature theory. It must be emphasized, once again, that the appearance of parasitic gaps in domains relatively inaccessible to ordinary extraction is rather different from an important distinction in language use. We will bring evidence in favor of the following thesis: a subset of English sentences interesting on quite independent grounds is not to be considered in determining problems of phonemic and morphological analysis.
see also: WikiPedia -- Chomskybot