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.
In the simplified AVM for the word “walks” below, the verb’s categorical information is divided into features that describe it (HEAD) and features that describe its arguments (VALENCE).
“Walks” is a sign of type word with a head of type verb. As an intransitive verb, “walks” has no complement but requires a subject that is a third person singular noun. The semantic value of the subject (CONTENT) is co-indexed with the verb’s only argument (the individual doing the walking). The following AVM for “she” represents a sign with a SYNSEM value that could fulfill those requirements.
Signs of type phrase unify with one or more daughters and propagate information upward. The following AVM is for a head-subj-phrase that requires two daughters: the head daughter (a verb) and a non-head daughter that fulfills the verb’s SUBJ constraints.
The end result is a sign with a verb head, empty subcategorization features, and a phonological value that orders the two daughters.
Although the actual grammar of HPSG is composed entirely of feature structures, linguists often use trees to represent the unification of signs where the equivalent AVM would be unwieldy.