Editing Phrase structure rules (section)

==Definition and examples==
Phrase structure rules are usually of the following form:

:<math>A \to B \quad C</math>

meaning that the [[constituent (linguistics)|constituent]] <math>A</math> is separated into the two subconstituents <math>B</math> and <math>C</math>. Some examples for English are as follows:

:<chem>S -> NP \quad VP</chem>
:<chem>NP -> (Det) \quad N1</chem>
:<chem>N1 -> (AP) \quad N1 \quad (PP)</chem>

The first rule reads: A S ([[Sentence (linguistics)|sentence]]) consists of a NP ([[noun phrase]]) followed by a VP ([[verb phrase]]). The second rule reads: A noun phrase consists of an optional Det ([[determiner]]) followed by a N (noun). The third rule means that a N (noun) can be preceded by an optional AP ([[adjective phrase]]) and followed by an optional PP ([[prepositional phrase]]). The round brackets indicate optional constituents.

Beginning with the sentence symbol S, and applying the phrase structure rules successively, finally applying replacement rules to substitute actual words for the abstract symbols, it is possible to generate many proper sentences of English (or whichever language the rules are specified for). If the rules are correct, then any sentence produced in this way ought to be grammatically (syntactically) [[grammaticality|correct]]. It is also to be expected that the rules will generate syntactically correct but [[semantics|semantically]] nonsensical sentences, such as the following well-known example:

::[[Colorless green ideas sleep furiously]]

This sentence was constructed by [[Noam Chomsky]] as an illustration that phrase structure rules are capable of generating syntactically correct but semantically incorrect sentences. Phrase structure rules break sentences down into their constituent parts. These constituents are often represented as [[parse tree|tree structures]] ([[dendrogram]]s). The tree for Chomsky's sentence can be rendered as follows:

:[[image:cgisf-tgg.svg|300px|Colorless green ideas sleep furiously.]]

A constituent is any word or combination of words that is dominated by a single node. Thus each individual word is a constituent. Further, the subject NP ''Colorless green ideas'', the minor NP ''green ideas'', and the VP ''sleep furiously'' are constituents. Phrase structure rules and the tree structures that are associated with them are a form of [[immediate constituent analysis]].

In [[transformational grammar]], systems of phrase structure rules are supplemented by transformation rules, which act on an existing syntactic structure to produce a new one (performing such operations as [[negation (linguistics)|negation]], [[passivization]], etc.). These transformations are not strictly required for generation, as the sentences they produce could be generated by a suitably expanded system of phrase structure rules alone, but transformations provide greater economy and enable significant relations between sentences to be reflected in the grammar.