Feature structures are a way of representing partial information about some linguistic object - or placing constraints on it

Unification can be seen as a way of merging the information in each structure - or describing objects which satisfy both constraints

Subsumption: when two structures are unified, each structure is more general than the result; thus, each structure subsumes the result. "A less specific (more abstract) feature structure subsumes an equally or more specific one."

PATR-II (Shieber 1986): Augment a CFG with feature specifications which place constraints on the elements in the rule.

Agreement:

(1) This flight serves breakfast.

(2) Does this flight serve breakfast?

(3) Do these flights serve breakfast?

S -> NP VP <NP Agreement> = <VP Agreement>

S -> Aux NP VP <Aux Agreement> = <NP Agreement>

NP -> Det Nominal <Det Agreement> = <Nominal Agreement>

Heads:

Any constituent can be thought of as composed of one head and one or more modifiers. Modifiers agree with the head; and the constituent as a whole agrees with its head ("feature percolation").

A noun phrase is plural if its head noun is plural; the determiner is plural if the head noun is plural.

In a PATR grammar, certain features ("head features") will be introduced by a lexical item and copied to other constituents via feature specifications on the PS rules.

Note that for particular constituent types, what is the head is controversial. Chomsky thinks the determiner is the head of the noun phrase:

(4) this flight serves breakfast

(5) this serves breakfast

(6) *flight serves breakfast

Subcategorization:

We can use features to encode a list of complements to a verb. For instance:

Verb -> leaves

<Verb Head Agreement Number> = SG

<Verb Head Subcat First Cat> = NP

<Verb Head Subcat Second Cat> = PP

<Verb Head Subcat Third> = End

Some verbs place feature restrictions on their complements. For instance, "want" takes an infinitive verb phrase:

<Verb Head Subcat First Cat> = VP

<Verb Head Subcat First Form> = Infinitive