Catalogue of Artificial Intelligence Techniques


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Tree-Adjoining Grammars

Aliases: TAG

Keywords: LFG, Lexical Functional Grammar

Categories: Natural Language

Author(s): Nicolas Nicolov

Tree-Adjoining Grammar (TAG) is a grammatical (and mathematical) formalism which constitutes a tree generating system (as opposed to string generating systems such as Context-free Grammars). TAG postulates the existence of a finite set of elementary trees out of which bigger syntactic trees can be built. Elementary trees are divided into initial trees and auxiliary trees--initial trees represent minimal syntactic structures while auxiliary trees correspond to minimal recursive structures. Bigger trees can be built by means of the operation of adjoining. Adjoining takes a recursive structure (auxiliary tree) and `inserts' it into another tree (either an initial tree or a tree derived by previous adjoining). Two important properties of TAGs which enable them to characterise the strong generative capacity of grammars (that is, their capacity to characterise the structural descriptions associated with sentences) are:

TAGs belong to the so-called mildly Context-sensitive Grammars. Thus, TAGs are more adequate for characterising various phenomena which require more formal power than Context-free Grammars and formalisms based on them.


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