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 called attribute value matrice (AVM)

The matrix has two columns. One for the feature names and the other for the values. In this sense a feature structure is a list lists of key-value pairs. The value might be atomic or another feature structure. This leads to another notation for feature structures - the use of trees. In fact some systems use S-expression to represent feature structures.