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

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

$S\; \backslash to\; NP\; \backslash quad\; VP$

$NP\; \backslash to\; Det\; \backslash quad\; N1$

$N1\; \backslash to\; (AP)\; \backslash quad\; N1\; \backslash quad\; (PP)$

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 can be diagrammed as a phrase tree, as below:

where S represents a grammatical sentence.

Alternative approaches

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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.

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(ADV))

And which would express the following conceptual content

X DOES Y IN THE MANNER OF Z

Though they are noncompositional, such models are monotonic. This approach is highly developed within Construction grammar, and has had some influence in Head-Driven Phrase Structure Grammar and Lexical functional grammar.

See also

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• Context free grammar
• Phrase structure grammar

Grammar frameworks