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 like this:

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

Here we have the two features category and agreement'. Category has the value noun phrase whereas the value of agreement is indicated by another feature structure whith the features number and person beeing singular and third.

This particular notation is sometimes called attribute value matrices (AVM) in the literature.

But as the the examples shows that the term matrix is misleading in this context. The feature structures are not matrices in the mathematical sense of the word. Basically we have lists of key-value pairs which are written down in a rectangualar-shaped way.