tree-adjoining grammar (TAG)

A type of FORMAL GRAMMAR which recognizes TREES as PRIMITIVE elements (elementary trees), combining these into larger structures; also called tree-adjunction grammar. Elementary trees are of two kinds: initial trees, which contain the basic PHRASAL elements of simple SENTENCES, without any RECURSION; and auxiliary trees, which represent recursive structures. The tag FORMALISM makes use of the operations of SUBSTITUTION (in which a ROOT NODE from one tree is merged with a non-terminal node in another, to produce a new tree) and ADJUNCTION (in which an auxiliary tree is attached to a non-terminal node in an initial tree). TAGs were devised by US computer scientist Aravind K. Joshi (b. 1929) and colleagues. They are weakly equivalent to CONTEXT-free grammars.