One versus more than one is among the most widespread grammatical distinctions. Number is a grammatical category that expresses contrasts such as singular and plural, and in some languages dual, trial, or paucal.

Number may appear on nouns, pronouns, verbs, and adjectives through agreement. English marks most noun plurals with -s but retains irregular forms. Some languages optionally mark number or restrict it to human referents.

Semantic number and grammatical number sometimes diverge, as with collective nouns or pluralia tantum. Acquisition and typology both pay close attention to number systems because they interface tightly with quantification and agreement. Teaching materials introduce number early as a high-frequency grammatical contrast.

• Grammatical contrast of singular, plural, and related values
• Marked on nouns and often via agreement
• May include dual or other categories
• Interfaces with quantification and semantics