Head-Driven Phrase Structure Grammar (HPSG) is a constraint-based theory of grammar that models linguistic structures through typed feature structures and principles governing the interaction of syntactic, semantic, and morphological information ^[1][2]. In HPSG, lexical items contain detailed information about their grammatical properties and combinatorial requirements, while phrase structure is constrained by general principles that specify how constituent features are shared and combined. The framework treats grammatical knowledge as a system of interacting constraints rather than relying primarily on transformational derivations, integrating lexical information and hierarchical syntactic representation within a unified formal model ^[1][2].