In languages with more flexible word order than English, non-projective dependencies are more frequent.
Various criteria have been proposed for deciding what is the head H and what is the dependent D in a construction C. Some of the most important are the following:
1. H determines the distribution class of C; or alternatively, the external syntactic properties of C are due to H.
2. H determines the semantic type of C.
3. H is obligatory while D may be optional.
4. H selects D and determines whether it is obligatory or optional.
5. The morphological form of D is determined by H (e.g., agreement or case government).
When we say in a phrase structure grammar that the immediate constituents of a PP are P and NP, we are implicitly appealing to the head/dependent distinction. A prepositional phrase is a phrase whose head is a preposition; moreover, the NP is a dependent of P. The same distinction carries over to the other types of phrase that we have discussed. The key point to note here is that although phrase structure grammars seem very different from dependency grammars, they implicitly embody a recognition of dependency relations. Although CFGs are not intended to directly capture dependencies, more recent linguistic frameworks have increasingly adopted formalisms which combine aspects of both approaches.
Let us take a closer look at verbs and their dependents. The grammar in Example 8-2 correctly generates examples like (12).
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