Dependency Structures and Transformational Rules

The concept head (of a phrase) is frequently invoked in transformational theory, but it has not been formalized and is perhaps not formalizable within the framework of a phrase-structure categorial component. However, if dependency rules generate the structured strings of categories, a head or governor is formally specified for each phrase, thus supplying additional information needed to determine the applicability of some transformations. Moreover, the problem of pruning inappropriately labeled or vacuous non-terminal nodes, which arises when transformations are applied to structures generated by phrase-structure rules, does not arise when they are applied to dependency structures. Although dependency rules are not well-adapted to generating structure-sensitive sets of strings, there is evidence that the structure-sensitive properties of language are more appropriately accounted for by transformational rules, leaving the catégorial component both context-free and structure-free.