GPSG

Generalized Phrase Structure Grammar (GPSG) is a formal framework for describing natural language syntax, developed in the early 1980s by Gerald Gazdar, Ewan Klein, Geoffrey Pullum, and Ivan Sag. It was presented most prominently in their 1985 book, Generalized Phrase Structure Grammar, and became influential in the study of computational linguistics and theoretical syntax.

GPSG extends context-free grammar by organizing linguistic knowledge into a small set of generalized phrase structure rules and a rich system of feature structures. The core idea is to separate the combinatory rules that build phrases from the grammatical information carried by features such as subcategorization, agreement, and selectional restrictions. Constraints are expressed through metarules that govern how features are inherited and unified, enabling the description of long-distance dependencies and cross-linguistic generalizations without ad hoc movement operations.

The architecture typically distinguishes a phrase-structure layer from a separate layer of feature geometry, with lexical entries contributing specific constraints via lexical rules. This design aims to provide a compact, highly systematic account of syntax that can be applied across languages while remaining explicit and testable.

Impact and reception: GPSG influenced subsequent constraint-based grammars, most notably the development of Head-Driven Phrase Structure Grammar (HPSG). While GPSG itself is less widely used in practical NLP today, its ideas—metarules, feature structures, and a unified treatment of syntactic constraints—shaped later theoretical work and informed the broader landscape of grammar formalisms.