The dual number existed in Proto-Indo-European, persisted in many of its descendants, such as Ancient Greek and Sanskrit, which have dual forms across nouns, verbs, and adjectives, and Gothic, which used dual forms in pronouns and verbs. It can still be found in a few modern Indo-European languages such as Scottish Gaelic, Slovenian, and Sorbian. The majority of modern Indo-European languages, however, only show residual traces of the dual whose function has since been replaced by simple plural, as is evident in the English distinctions: both vs. all, either vs. any, neither vs. none, and so on.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 people who had already been determined in the conversation.
Many Semitic languages also have dual number. For instance, in Arabic, ان -ān, is added to the end of any noun to indicate that it is dual (regardless of how the plural is being formed).
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, 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, 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 you want to stress out 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.
Although relatively few languages have the dual number and most have no number or only singular and plural, using different words for groups of two and groups greater than two is not uncommon. English has words distinguishing dual vs. plural number, including: both/all, either/any, neither/none, between/among, former/first, and latter/last. Japanese, which has no grammatical number, also has words dochira (which of the two) and dore (which of the three or more), etc.