Overview of Grammatical number

Grammatical number is a morphological category characterized by the expression of quantity through inflection or agreement. As an example, consider the English sentences below:

That apple on the table is fresh.
Those two apples on the table are fresh.

The number of apples is marked on the noun — "apple", singular number (one item) vs. "apples", plural number (more than one item) —, on the demonstrative, "that/those", and on the verb, "is/are". Note that, especially in the second sentence, this information can be considered redundant, since quantity is already indicated by the numeral "two".

A language has grammatical number when its nouns are subdivided into morphological classes according to the quantity they express, such that:

1. Every noun belongs to a single number class. (Number partitions nouns into disjoint classes.)
2. Noun modifiers (such as adjectives) and verbs have different forms for each number class, and must be inflected to match the number of the nouns they refer to. (Number is an agreement category.)

This is the case in English: every noun is either singular or plural (a few, such as "fish", can be either, according to context), and at least some modifiers of nouns — namely the demonstratives, the personal pronouns, the articles, and verbs — are inflected to agree with the number of the nouns they refer to: "this car" and "these cars" are correct, while "*this cars" or "*these car" are ungrammatical.

Not all languages have number as a grammatical category. In those that do not, quantity must be expressed either directly, with numerals, or indirectly, through optional quantifiers. However, many of these languages compensate for the lack of grammatical number with an extensive system of measure words.