Editing Phrase structure rules (section)

==Alternative approaches==
===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).

===Representational grammars===
A number of representational phrase structure theories of grammar never acknowledged phrase structure rules, but have pursued instead an understanding of sentence structure in terms the notion of [[Schema (psychology)|schema]]. Here phrase structures are not derived from rules that combine words, but from the specification or instantiation of syntactic schemata or configurations, often expressing some kind of semantic content independently of the specific words that appear in them. This approach is essentially equivalent to a system of phrase structure rules combined with a non[[compositionality|compositional]] [[semantics|semantic]] theory, since grammatical formalisms based on rewriting rules are generally equivalent in power to those based on substitution into schemata.

So in this type of approach, instead of being derived from the application of a number of phrase structure rules, the sentence ''Colorless green ideas sleep furiously'' would be generated by filling the words into the slots of a schema having the following structure:

::[NP[ADJ N] VP[V] AP<nowiki>[ADV]]</nowiki>

And which would express the following conceptual content:

::X DOES Y IN THE MANNER OF Z

Though they are non-compositional, such models are monotonic. This approach is highly developed within [[Construction grammar]]<ref>Concerning Construction Grammar, see Goldberg (2006).</ref> and has had some influence in [[Head-Driven Phrase Structure Grammar]]<ref>Concerning Head-Driven Phrase Structure Grammar, see Pollard and Sag (1994).</ref> and [[lexical functional grammar]],<ref>Concerning Lexical Functional Grammar, see Bresnan (2001).</ref> the latter two clearly qualifying as phrase structure grammars.