phrase-structure grammar (PSG)

A type of GRAMMAR discussed by Noam Chomsky in his book Syntactic Structures (1957) as an illustration of a GENERATIVE DEVICE. Phrase-structure grammars contain RULES (PS-rules) which are capable not only of generating STRINGS of LINGUISTIC ELEMENTS, but also of providing a CONSTITUENT analysis of the strings, and hence more information than FINITE-STATE GRAMMARS. They are not, however, as POWERFUL as TRANSFORMATIONAL grammars, as the latter are more capable of displaying certain types of INTUITIVE relationship between SENTENCES, and may ultimately be demonstrable as SIMPLER. In a related sense, the phrase-structure component of a transformational grammar specifies the HIERARCHICAL structure of a sentence, the linear sequence of its constituents, and indirectly (through the notion of DOMINANCE) some types of SYNTACTIC RELATIONs.

The main difference between the phrase-structure grammars (PSGs) of Chomsky as opposed to the IMMEDIATE-CONSTITUENT analysis of earlier linguists is that Chomsky’s MODEL is FORMALIZED as a system of generative rules, and aims to avoid the emphasis on DISCOVERY PROCEDURES characteristic of the earlier approach. In their original formulation, PSGs took the form of a set of REWRITE RULES (with the abbreviations expanded here), such as:

Sentence ⇒ Noun Phrase + Verb Phrase

Verb phrase ⇒ Verb + Noun Phrase

Noun Phrase ⇒ Determiner + Noun

Various distinctions have been made in the classification of phrase-structure grammars, of which the main division is into context-free and context-sensitive types: a grammar consisting wholly of context-free rules (rules which are of the form ‘Rewrite X as Y’, i.e. regardless of CONTEXT) is much less powerful than a grammar containing context-sensitive rules (rules which are of the form ‘Rewrite X as Y in the context of Z’). In later linguistic theory several approaches to syntax were developed which are equivalent to PSGs, but do not employ PS rules, and are thus able to capture generalizations missed by ordinary PSGs. Examples include GENERALIZED PHRASE-STRUCTURE GRAMMAR and HEAD-DRIVEN PHRASE-STRUCTURE GRAMMAR. The MINIMALIST PROGRAMME introduces a major simplification of the notion (bare phrase structure).