Feature structure

The value of an attribute may be either 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.

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

In this sense a feature structure is a list of key-value pairs.

In fact, some systems (such as PATR-II) use S-expressions to represent feature structures.