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In [[phrase structure grammar]]s, such as [[generalised phrase structure grammar]], [[head-driven phrase structure grammar]] and [[lexical functional grammar]], a '''feature structure''' is essentially a set of [[attribute–value pair]]s. For example, the attribute named ''number'' might have the value ''singular''. The value of an attribute may be either [[Lisp atom|atomic]], e.g. the symbol ''singular'', or complex (most commonly a feature structure, but also a list or a set).

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_(computer_science)|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:

<math>\begin{bmatrix} \mbox{category} & noun\ phrase\\ \mbox{agreement} & \begin{bmatrix} \mbox{number} & singular \\ \mbox{person} & third \end{bmatrix} \end{bmatrix}</math>

Here there are the two features ''category'' and ''agreement''. ''Category'' has the value ''noun phrase'' whereas the value of ''agreement'' is indicated by another feature structure with the features ''number'' and ''person'' being ''singular'' and ''third''.

This particular notation is called ''attribute value matrix'' (AVM).

The [[matrix (mathematics)|matrix]] has two columns, one for the feature names and the other for the values. In this sense a feature structure is a list of key-value pairs. The value might be atomic or another feature structure. This leads to another notation for feature structures: the use of [[Tree (data structure)|tree]]s. In fact, some systems (such as [[PATR-II]]) use [[S-expression]]s to represent feature structures.

==External links==
*[http://cs.union.edu/~striegnk/courses/nlp-with-prolog/html/node79.html Feature Structures] section of an online [[Prolog]] course
*[http://www.tei-c.org/release/doc/tei-p5-doc/en/html/FS.html Feature Structures] in Text Encoding for Interchange ([[Text Encoding Initiative|TEI]])

[[Category:Grammar]]

Feature structure - Revision history

imported>Arademaker: fix link