Editing Phrase structure rules (section)

===Constituency vs. dependency===
Phrase structure rules as they are commonly employed result in a view of sentence structure that is ''constituency-based''. Thus, grammars that employ phrase structure rules are ''constituency grammars'' (= [[phrase structure grammar]]s), as opposed to ''[[dependency grammar]]s'',<ref>The most comprehensive source on dependency grammar is Ágel et al. (2003/6).</ref> which view sentence structure as ''dependency-based''. What this means is that for phrase structure rules to be applicable at all, one has to pursue a constituency-based understanding of sentence structure. The constituency relation is a one-to-one-or-more correspondence. For every word in a sentence, there is at least one node in the syntactic structure that corresponds to that word. The dependency relation, in contrast, is a one-to-one relation; for every word in the sentence, there is exactly one node in the syntactic structure that corresponds to that word. The distinction is illustrated with the following trees:

:[[File:Phrase structure rules.jpg|Phrase structure rules: Constituency vs. dependency]]

The constituency tree on the left could be generated by phrase structure rules. The sentence S is broken down into smaller and smaller constituent parts. The dependency tree on the right could not, in contrast, be generated by phrase structure rules (at least not as they are commonly interpreted).