History
TAG originated in investigations by Joshi and his students into the family of adjunction grammars (AG), the "string grammar" of Zellig Harris. AGs handle endocentric properties of language in a natural and effective way, but do not have a good characterization of exocentric constructions; the converse is true of rewrite grammars, or phrase-structure grammar (PSG). In 1969, Joshi introduced a family of grammars that exploits this complementarity by mixing the two types of rules. A few very simple rewrite rules suffice to generate the vocabulary of strings for adjunction rules. This family is distinct from the Chomsky hierarchy but intersects it in interesting and linguistically relevant ways.
Read more about this topic: Tree-adjoining Grammar
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