Comparing these examples with their parasitic gap counterparts in (96) and (97), we see that an important property of these three types of EC may remedy and, at the same time, eliminate an important distinction in language use. From C1, it follows that the appearance of parasitic gaps in domains relatively inaccessible to ordinary extraction does not affect the structure of a general convention regarding the forms of the grammar. To characterize a linguistic level L, a subset of English sentences interesting on quite independent grounds delimits the requirement that branching is not tolerated within the dominance scope of a complex symbol. Notice, incidentally, that the systematic use of complex symbols is not to be considered in determining a stipulation to place the constructions into these various categories. It may be, then, that relational information cannot be arbitrary in problems of phonemic and morphological analysis.
see also: WikiPedia -- Chomskybot