Information-Based Syntax and Semantics. Vol. 1: Fundamentals.
Pollard, Carl and Sag, Ivan A.
CSLI. CSLI Lecture Notes. 13. Stanford. 1987.

Head-Driven Phrase Structure Grammar.
Pollard, Carl and Sag, Ivan A.

HPSG is a constraint-based, lexicalist approach to grammatical theory that seeks to model human languages as systems of constraints on typed feature structures. Lexical information is organized in terms of multiple inheritance hierarchies that allow complex properties of words to be derived from the logic of the lexicon. Phrasal types are also treated in terms of multiple inheritance hierarchies that allow generalizations about diverse construction types to be factored into various cross-cutting dimensions.