On our assumptions, a case of semigrammaticalness of a different sort appears to correlate rather closely with the extended c-command discussed in connection with (34). If the position of the trace in (99c) were only relatively inaccessible to movement, the notion of level of grammaticalness is necessary to impose an interpretation on a descriptive fact. Summarizing, then, we assume that the systematic use of complex symbols is not to be considered in determining problems of phonemic and morphological analysis. I suggested that these results would follow from the assumption that most of the methodological work in modern linguistics is not subject to the requirement that branching is not tolerated within the dominance scope of a complex symbol. It must be emphasized, once again, that this analysis of a formative as a pair of sets of features delimits the levels of acceptability from fairly high (eg (99a)) to virtual gibberish (eg (98d)).
see also: WikiPedia -- Chomskybot