Feature structure

In phrase structure grammars, such as generalised phrase structure grammar, head-driven phrase structure grammar and lexical functional grammar, a feature structure is essentially a list of property types with values. For example the property named number might have the value singular. The value of a property may be either atomic, e.g. the symbol singular, or may be a feature structure itself.

A feature structure can be represented as a 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. In most theories (e.g. HPSG), operations are strictly speaking defined over equations describing feature structures and not over feature structures themselves, though feature structures are usually used in informal exposition.

Often, feature structures are written as attribute value matrices (AVM), like this:

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

It is important to note that the term matrix might be misleading in this context. The feature structures are not matrices in the strict sense of the word. The term matrix in this context signifies that the structure is written in a rectangualar area.