Abstract
AbstractMultiple context‐free grammars (mcfg's) and head grammars (hg's) are subclasses of generalized context‐free grammars introduced by Pollard to describe the syntax of natural languages. In this paper, the following properties of mcfg's and hg's are shown: (1) the class HL of languages generated by hg's is the same as that generated by the tree adjoining grammars introduced by Joshi et al.; (2) HL is the same as that generated by left‐wrapping hg's (or right‐wrapping hg's) which is a proper subclass of hg's; (3) HL is properly included in the one generated by 2‐mcfg's, which is a proper subclass of mcfg's. As a corollary of (1), it is also shown that HL is a substitution closed full AFL.
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