Phrase structure rules

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Phrase-structure rules are a way to describe a given language's syntax. They are 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 commonly used in transformational grammar (TGG), although they were not an invention of TGG; rather, early TGG's added 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 - except in computer science, where it is known as just a grammar, usually context-free.


Phrase structure rules are usually of the form A \to B \quad C, meaning that the constituent A is separated into the two subconstituents B and C.

Some examples correct inter alia for natural English language are:

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, PP

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 can be diagrammed as a phrase tree, as below:

Colorless green ideas sleep furiously.

where S represents a grammatical sentence. The theory of antisymmetry proposed in the early '90s by Richard Kayne is an attempt to derive phrase structure from a single axiom. A phrase tree can be represented by a ultrametric, see Mark D. Roberts

Alternative approaches

A number of theories of grammar dispense with the notion of phrase structure rules and operate with the notion of schema instead. 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 noncompositional semantic theory, since grammatical formalisms based on rewriting rules are generally equivalent in power to those based on substitution into schemata.

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