Feature structure

In Phrase Structure Grammars, such as the Generalised Phrase Structure Grammar and the Head-Driven Phrase Structure Grammar, a feature structure is basically a list of variable assignments. As is usual, the variables have names and values, e.g. the variable named number might be assigned the value singular. The value of the variable may be either atomic, e.g. the symbol singular, or may be a feature structure itself. Often, feature structures are written as attribute value matrices, like this:

${\begin{bmatrix}{\mbox{category}}&noun\ phrase\\{\mbox{agreement}}&{\begin{bmatrix}{\mbox{number}}&singular\\{\mbox{person}}&third\end{bmatrix}}\end{bmatrix}}$

Feature structures may also be written as directed acyclic graph (DAG) with the nodes corresponding to the variable values and the paths to the variable names. Operations defined on feature structures, e.g. unification, are used extensively in Phrase Structure Grammars.