Dual

Dual can be used to connotate something being both at the same instant (i.e,. being twofold), such as in duality. The duality of a phenonomen is characterized by the fact that some properties can be explained best by one model and others by another model. (see wave-particle duality)

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In some languages, in addition to the singular and plural there is also a dual, which is used when exactly two people or things are meant. The plural in these languages is used only for groups greater than two. This is known as the dual grammatical number.

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The word dual is used in mathematics, as in dual space, dual polyhedron and dual numbers, for example.

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Dual is a brand name for Hifi[?] equipment, mostly know for vinyl record players.