Editing Tree-adjoining grammar (section)

==Description==
The rules in a TAG are trees with a special leaf node known as the ''foot node'', which is anchored to a word. 
There are two types of basic trees in TAG: ''initial'' trees (often represented as '<math>\alpha</math>') and ''auxiliary'' trees ('<math>\beta</math>').  Initial trees represent basic valency relations, while auxiliary trees allow for recursion.<ref name="jurafsky-martin2000">{{cite book
 | last = Jurafsky
 | first = Daniel
 | author-link = Daniel Jurafsky
 |author2=James H. Martin
  | title = Speech and Language Processing
 | year = 2000
 | pages = 354
 | publisher = Prentice Hall
 | location = Upper Saddle River, NJ }}</ref>
Auxiliary trees have the root (top) node and foot node labeled with the same symbol.
A derivation starts with an initial tree, combining via either ''substitution'' or ''adjunction''.   Substitution replaces a frontier node with another tree whose top node has the same label. The root/foot label of the auxiliary tree must match the label of the node at which it adjoins. Adjunction can thus have the effect of inserting an auxiliary tree into the center of another tree.<ref name="joshi-rambow2003"/>

Other variants of TAG allow [[multi-component tree adjoining grammars|multi-component trees]], trees with multiple foot nodes, and other extensions.