TAG originated in investigations by Joshi and his students into the family of adjunction grammars (AG),[1] the "string grammar" of Zellig Harris.
[2] AGs handle exocentric properties of language in a natural and effective way, but do not have a good characterization of endocentric 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.
This family is distinct from the Chomsky-Schützenberger hierarchy but intersects it in interesting and linguistically relevant ways.
A derivation starts with an initial tree, combining via either substitution or adjunction.
This type of processing can be represented by an embedded pushdown automaton.
For these reasons, tree-adjoining grammars are often described as mildly context-sensitive.
These grammar classes are conjectured to be powerful enough to model natural languages while remaining efficiently parsable in the general case.
A lexicalized grammar for English has been developed by the XTAG Research Group of the Institute for Research in Cognitive Science at the University of Pennsylvania.