12.7.1 The Relationship Between Dependencies and Heads

The reader may have noticed the similarity between dependency graphs like Fig. 12.14 and head structures like Fig. 12.12. In fact an (unlabeled) dependency graph can be automatically derived from a context-free parse by using the head rules; here's an algorithm from Xia and Palmer (2001):

1. Mark the head child of each node in a phrase structure, using the head percolation table.

2. In the dependency structure, make the head of each non-head-child depend on the head of the head-child.

This algorithm applied to the parse tree in Fig. 12.15 would produce the dependency structure in Fig. 12.16.

We will return to the discussion of heads and dependencies when we discuss lexicalized parsing in Ch. 14 and again when we introduce head features and subcategorization in Ch. 16.