1. Head-driven Phrase Structure Grammar (HPSG)
An integrated theory of natural language syntax and semantics

2. Definition
HPSG is a ‘sign based’ grammar, where phonological, syntactic and semantic information is integrated into a formally precise description of linguistic objects using feature structures, developed by Ivan Sag and Carl Pollard in the mid 80s.
Universal grammar:
a) linguistic signs;
b) combination principles.

3. Main features Features and values characterise linguistic objects;
Structure sharing allows to say that certain values in a feature structure are identical;
Valence information is represented in lists in a complex description of the head;
Types allows for classification of (linguistic) objects.

4. Framework
HPSG assumes features structures as models of linguistic objects.
Feature structures are called AVM (Attribute Value Matrix);
AVM consists of feature value pairs;
The values can be atomic or feature descriptions;
Every feature structure is of a certain type;
Types are ordered in hierarchies ;
Hierarchies have the most general type at the top and the most specific at the bottom;
case
nom gen dat acc voc
Figure 1: Subtypes of case in a Romanian grammar

5. Attribute value matrice - AVM
word
PHON <‘talks’>
synsem
category
HEAD verb
CAT VALENCE C AT HEAD noun
SUBJ ref-index
SYNSEM PER 3rd
CONT NUM sing
COMP <>
content
CONT TALKER 1 1

6. AVM’s translation AVM for the word ‘talks’;
The verb categorical information is divided into features that describe it, HEAD, and features that describe its arguments, VALENCE.
Talks is a sign of type word with a head of type verb.
Intransitive verb with no complement requires a subject that is a third person singular noun.
The semantic value of the subject is co-indexed with the verb’s only argument (the individual doing the talking).

7. Motivations for HPSG
Computer processable grammars that scale up and can be implemented;
Most wide-spread grammatical framework employed in computational linguistics;
A must for everyone working on natural language processing;
Increased precision;
Framework for integration;
Psycholinguistic plausibility.

8. Implementations
Various parser based on the HPSG formalism have been written, e.g. Enju
Currently there are grammars for German, Mandarin, Chinese, Maltese and Persian that share a common core and are publicly available;
Large HPSG grammars of various languages are being developed in the Deep Linguistic Processing with HPSG initiative;
The Babel system, a system of analysing written language

9. Bibliography http://hpsg.stanford.edu
C Pollard, IA Sag books.google.com
Carl Pollard, Ivan A. Sag (1987): Information-based Syntax and Semantics. Volume 1: Fundamentals. Stanford: CSLI Publications.