A feature in Unification Grammar is an "attribute-value pair". A feature matrix is a list of attribute-value pairs.

A noun phrase like "that flight" might have the following feature matrix:

The value of an attribute can be another feature matrix:

"Indices" can be used which will force features in different parts of the structure to have matching values. The following structure requires the head of the sentence (the verb) to agree in number and person with the subject.