On Notional Agreement, the Majority Speak

Most English speakers know the basic rule of subject-verb agreement: a singular noun takes a singular verb, and a plural noun takes its corresponding plural.

The student is in the cafeteria.

The students are in the cafeteria.

This is pretty straightforward: is is the third-person singular conjugation of the verb to be that agrees with student; are is the third-person plural conjugation of to be that agrees with the plural subject students.

The same rule applies when the construction is inverted:

There is a student in the cafeteria.

There are students in the cafeteria.

But there are times when the determination for what counts as "agreement" is not as obvious, because what sounds like a singular noun is really plural, or what sounds like a plural noun is essentially singular. This concept is known as notional agreement, otherwise known as notional concord or synesis.

"A crowd of revelers were approaching." Or 'was' it?

Simply put, notional agreement occurs when the agreement between a subject and its verb (or, in some instances, a pronoun and its antecedent) is determined by meaning rather than form.

For example, when you have a compound or plural subject that works as a singular unit, it sometimes sounds more "natural" for that subject to take a singular verb, in spite of formal rules to the contrary.

This would apply, for example, to nouns that are often combined and presented together:

There is leftover macaroni and cheese in the refrigerator.

Track and field is her favorite sport.

And to amounts presented abstractly as unit quantities:

Ten dollars is the cost of admission.

Is five miles too far to walk?

Two plus three makes five.

It also works for nouns that are spelled in a plural form but represent something with a singular nature. This is particularly common in the case of nouns referring to areas of study, like politics, civics, or economics:

Politics is best not discussed at the dinner table.

Simple economics determines how much something costs.

Alternately, you will frequently see instances where a plural verb is used with a singular noun that, because of its meaning and context, suggests a plurality. Such nouns include pair, trio, crowd, family, crew, mob, generation, and committee. You might see a sentence such as "The pair were seen leaving in a gray car" or "The crew were preparing for the launch," where what are normally singular subject nouns (pair and crew) are paired with a plural verb (were).

Ultimately, context comes into play, with the sentence usually offering some kind of information that emphasizes the plural essence of what is technically a singular noun. By "The pair were seen leaving in a gray car," one can intuit that two people were seen; similarly, "The crew were preparing for the launch" brings to mind many people working together, suggesting a plurality, and it's that notion that leads a speaker to prefer a plural verb.

More common are constructions that "set aside" a singular noun from its plural members (as in the model "a [collective noun] of [member nouns]). With these subjects, speakers and writers will frequently opt to express the verb in the plural:

A majority of the voters support the amendment.

There are a handful of good reasons for us to go.

A committee of volunteers were selected.

In addition to notional agreement, there's a second principle at play here that makes the use of a plural verb sound more "correct" than the singular verb, and that is what is known as the principle of proximity. That means, for example, that in a construction such as "a crowd of revelers," one might be more inclined to select a verb form that agrees with the plural noun that is closer in the sentence to the verb (revelers) rather than the more distant singular noun (crowd):

A crowd of revelers were approaching.

Notional agreement is something to which we don't often pay notice because it's almost instinctive, a part of our regular speaking habits. And it's not a set rule in its own right, but rather a matter of preference, and it's more common in British English than American English. If you preferred to say "a crowd of revelers was approaching," you wouldn't be wrong.