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Head-driven phrase structure grammar (HPSG) is a highly lexicalized, non-derivational generative grammar theory developed by Carl Pollard and Ivan Sag (1985). It is the immediate successor to generalized phrase structure grammar. HPSG draws from other fields such as computer science (data type theory and knowledge representation) and uses Ferdinand de Saussure's notion of the sign. It uses a uniform formalism and is organized in a modular way which makes it attractive for natural language processing.

An HPSG grammar includes principles and grammar rules and lexicon entries which are normally not considered to belong to a grammar. The formalism is based on lexicalism. This means that the lexicon is more than just a list of entries; it is in itself richly structured. Individual entries are marked with types. Types form a hierarchy.

The basic type HPSG deals with is the sign. Words and phrases are two different subtypes of sign. A word has two features: [PHON] (the sound, the phonetic form) and [SYNSEM] (the syntactic and semantic information), both of which are split into subfeatures. Signs and rules are formalized as typed feature structures.

Various parsers based on the HPSG formalism have been written and optimizations are currently being investigated. An example of a system analyzing German sentences is provided by the University of Bremen^[1]. For Dutch, the wide-coverage dependency parser Alpino ^[2] has been developed by the University of Groningen.

Large HPSG grammars of various languages are being developed in the DELPH-IN ^[3] collaboration network. Wide-coverage grammars of German ^[4], English ^[5] and Japanese ^[6] are available under an open-source license.

• Generalised phrase structure grammar
• Lexical-functional grammar
• Syntax
• Transformational grammar

• Carl Pollard, Ivan A. Sag (1987): Information-based Syntax and Semantics. Volume 1: Fundamentals. Stanford: CSLI Publications.
• Carl Pollard, Ivan A. Sag (1994): Head-Driven Phrase Structure Grammar. Chicago: University of Chicago Press. ([1])
• Ivan A. Sag, Thomas Wasow, Emily Bender (2003): Syntactic Theory: a formal introduction, Second Edition. Chicago: University of Chicago Press. ([2])

• Head-driven phrase structure grammar, Stanford (includes on-line proceedings of an annual HPSG conference)
• Ohio State HPSG homepage
• Delph-In network for HPSG grammar development
• Summary
• Compares HPSG with alternatives, and offers historical perspective
• Bibliography of HPSG Publications

1. ^ http://www.cl.uni-bremen.de/~stefan/Babel/Interaktiv/
2. ^ http://www.let.rug.nl/~vannoord/alp/Alpino/
3. ^ http://www.delph-in.net/
4. ^ http://gg.dfki.de/
5. ^ http://www.delph-in.net/erg/
6. ^ http://www.delph-in.net/jacy