HPSG

Head-Driven Phrase Structure Grammar (HPSG) is a constraint-based grammatical framework developed by Carl Pollard and Ivan Sag in 1987. HPSG represents linguistic information (phonological, syntactic, semantic) in typed feature structures and derives well-formed expressions through constraints on these structures. Unlike movement-based theories, HPSG models word order and long-distance dependencies through feature sharing and principles of grammar. It has been extensively applied to modeling diverse language phenomena and remains influential in computational linguistics.