Phrase structure rules
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Phrase-structure rules were used in early transformational grammar (TGG) to describe a given language's syntax. They were used to break a natural language sentence down into its constituent parts (also known as syntactic categories) namely phrasal categories and lexical categories (aka parts of speech). Phrasal categories include the noun phrase, verb phrase, and prepositional phrase; lexical categories include noun, verb, adjective, adverb, and many others. Phrase structure rules were not an invention of TGG; rather, early TGG's defining characteristics were those systems which it had in addition to phrase structure rules (the most obvious example being transformations;see the page transformational grammar for an overview of the development of TGG.) A grammar which uses phrase structure rules is called a phrase structure grammar.
Definition
Phrase structure rules are usually of the form <math>A \to B \quad C<math>, meaning that the constituent <math>A<math> is separated into the two subconstituents <math>B<math> and <math>C<math>. Some examples are:
- <math>S \to NP \quad VP<math>
- <math>NP \to Det \quad N1<math>
- <math>N1 \to (AP) \quad N1 \quad (PP)<math>
The first rule reads: An S consists of an NP followed by a VP. This means A sentence consists of a noun phrase followed by a verb phrase. The next one: A noun phrase consists of a determiner followed by a noun.
Further explanations of the constituents: S, Det, NP, VP, AP
Associated with phrase structure rules is a famous example of a grammatically correct sentence. The sentence was constructed by Noam Chomsky as an illustration that syntactically but not semantically correct sentences are possible.
Colorless green ideas sleep furiously, which can be diagrammed into a phrase tree as below:
Where S represents a grammatical sentence.
This phrase tree can also be represented with the following Lisp S-expression:
- ((NP (ADJ colorless) (NP (ADJ green) (N ideas)))
- (VP (V sleep) (ADV furiously)))