Dual (abbreviated DU) is a grammatical number that some languages use in addition to singular and plural. When a noun or pronoun appears in dual form, it is interpreted as referring to precisely two of the entities (objects or persons) identified by the noun or pronoun acting as a single unit or in unison. Verbs can also have dual agreement forms in these languages.

The dual number existed in Proto-Indo-European and persisted in many of its descendants, such as Ancient Greek and Sanskrit, which have dual forms across nouns, verbs, and adjectives; Gothic, which used dual forms in pronouns and verbs; and Old English (Anglo-Saxon), which used dual forms in its pronouns. It can still be found in a few modern Indo-European languages such as Irish, Scottish Gaelic, Lithuanian, Slovene, and Sorbian languages.

The majority of modern Indo-European languages, including modern English, have lost the dual number through their development. Its function has mostly been replaced by the simple plural. They may however show residual traces of the dual, for example in the English distinctions: both vs. all, either vs. any, neither vs. none, and so on. A commonly used sentence to exemplify dual in English is "Both go to the same school." where both refers to two specific people who had already been determined in the conversation.

Many Semitic languages have dual number. For instance, in Hebrew יים‎- (-ayim) or a variation of it is added to the end of some nouns, e.g. some parts of the body (eye, ear, nostril, lip, hand, leg) and some time periods (minute, hour, day, week, month, year) to indicate that it is dual (regardless of how the plural is formed). A similar situation exists in classical Arabic, where ان -ān is added to the end of any noun to indicate that it is dual (regardless of how the plural is formed).

It is also present in Khoisan languages that have a rich inflectional morphology, particularly Khoe languages, as well as Kunama, a Nilo-Saharan language.

Many languages make a distinction between singular and plural: English, for example, distinguishes between man and men, or house and houses. In some languages, in addition to such singular and plural forms, there is also a dual form, which is used when exactly two people or things are meant. In many languages with dual forms, the use of the dual is mandatory as in some Arabic dialects using dual in nouns as in Hejazi Arabic, and the plural is used only for groups greater than two. However, the use of the dual is optional in some languages such as other modern Arabic dialects including Egyptian Arabic.

In other languages such as Hebrew, the dual exists only for words naming time spans (day, week, etc.), a few measure words, and for words that naturally come in pairs and are not used in the plural except in rhetoric: eyes, ears, and so forth.

In Slovene, the use of the dual is mandatory except for nouns that are natural pairs, such as trousers, eyes, ears, lips, hands, arms, legs, feet, kidneys, breasts, lungs, etc., for which the plural form has to be used unless one wants to stress that something is true for both one and the other part. For example, one says oči me bolijo ('my eyes hurt'), but if they want to stress that both their eyes hurt, they say obe očesi me bolita. When using the pronoun obe/oba ('both'), the dual form that follows is mandatory. But the use of "obe (both)" is not mandatory since "očesi (two eyes)" as it is, implies that one means both eyes.