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
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
An HPSG grammar includes principles and grammar rules
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
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
A Sample Grammar
HPSG generates strings by combining signs, which are
defined by their location within a type hierarchy and
by their internal feature structure, represented by attribute
value matrices (AVMs). 
Features take types or lists of types as their values,
and these values may in turn have their own feature structure.
Grammatical rules are largely expressed through the constraints
signs place on one another. A sign’s feature structure
describes its phonological, syntactic, and semantic properties.
In common notation, AVMs are written with features in
upper case and types in italicized lower case. Numbered
indices in an AVM represent token identical values.
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
“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
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
based on the HPSG formalism have been written and optimizations
are currently being investigated. An example of a system
is provided by the Freie
In addition the Grammar Group of the Freie
Universität Berlin provides open source grammars
that were implemented in the TRALE system. Currently there
are grammars for German,
that share a common core and are publicly available. For
the wide-coverage dependency parser Alpino 
has been developed at the University
Large HPSG grammars of various languages are being developed
in the DELPH-IN 
collaboration network. Wide-coverage grammars of German
are available under an open-source license.
Pollard, Carl; Ivan A. Sag. (1994). Head-driven phrase
structure grammar. Chicago: University of Chicago
Sag, Ivan A.; Thomas Wasow; & Emily Bender. (2003).
Syntactic theory: a formal introduction. 2nd
ed. Chicago: University of Chicago Press.
Babel-System: HPSG Interactive
Delph-In: Open-Source Deep Processing
Resource Grammar and Lexicon
- Deep Linguistic Processing with HPSG (DELPH-IN)