Syntactic Theory

The Lexicon

The lexicon contains a list of all words (or lexemes) of a language and gives syntactic, semantic and phonological information about each lexical entry.

In HPSG, the lexical head determines the structure of the phrase. Therefore, the lexicon is quite 'rich'. Because of this lexicalization, HPSG has relatively few grammar rules.
Most of this information is organized systematically by a type hierarchy and lexical rules. Each lexical entry is associated with a particular lexeme. By associating this lexeme with the appropriate type in the lexeme hierarchy, most specifications that are associated with this lexeme can be predicted. Therefore, the 'basic' lexical entry is quite streamlined. It only needs to provide a description of a phonological form, the particular predication in its semantic restriction, and any exceptional properties it might have. All other constraints are inherited via the lexeme hierarchy. Finally, lexical rules (inflectional rules) turn the lexemes into words, which are used to build syntactic structures.

Below is a typical 'basic' lexical entry for an English count noun as it is given in the lexicon. It only includes information that is not inherited from other types.

adapted from Sag/Wasow/Bender 2003, p. 239

The lexical entry's first member is its phonological description, the second member is a feature structure description, which gives some semantic information. Since the feature structure is of type cntn-lxm, the lexical entry inherits constraints from the type cntn-lxm and all of its supertypes. The following picture shows the type hierarchy that is assumed for nominal and verbal lexemes in English. As you can see, the type cntn-lxm is a subtype of - and therefore inherits constraints from - the types cn-lxm, infl-lxm and lexeme:

from Sag/Wasow/Bender 2003

The following is a lexical sequence that shows all of the constraints inherited through the type hierarchy:

adapted from Sag/Wasow/Bender 2003, p. 239

This lexical entry gives rise to a family of words, which are used to build phrasal structures. These words are derived by inflectional rules. For example, the Singular Noun Lexical Rule maps a nominal lexeme (as, for example, the countnoun lexeme above) into its corresponding singular form, and the Plural Noun Lexical Rule maps a nominal lexeme into its corresponding plural form. Both are lexical sequences whose second member is a feature structure of type word (and hence obeying the Argument Realization Principle). These words can be used as the daughters of phrase structure rules to build phrases and sentences.