 Grammatical agreement

Grammatical agreement is a formal relationship in which the form of one linguistic element requires a corresponding form of another. Grammatical number can participate in this relationship, but the features expressed and the elements involved vary among languages. Agreement is a constraint on connected grammatical elements, rather than a requirement that adjacent words merely resemble each other.^[ref]

Compare The key to the cabinets is missing with The keys to the cabinet are missing. In these English examples, the verb form tracks the number of the subject's head noun, not the closest noun. An explicit model can assign Number=Sing to key and require the corresponding agreement form is. Changing the embedded noun cabinet to cabinets leaves that requirement intact. Such contrasts provide testable predictions for a proposed syntax and show why a rule based only on the nearest preceding word fails.