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 keeps irregular forms. Some languages mark number optionally or only for human referents.

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