Nevertheless, relational information is, apparently, determined by the ultimate standard that determines the accuracy of any proposed grammar. However, this assumption is not correct, since most of the methodological work in modern linguistics is not to be considered in determining a parasitic gap construction. Conversely, this selectionally introduced contextual feature cannot be arbitrary in the traditional practice of grammarians. Let us continue to suppose that the notion of level of grammaticalness is rather different from the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)). Thus the descriptive power of the base component is to be regarded as the requirement that branching is not tolerated within the dominance scope of a complex symbol.
see also: WikiPedia -- Chomskybot