R, or r, is the eighteenth letter of the English alphabet.

R or r may also refer to:

Science

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Biology and medicine

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Astronomy

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Physics

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Temperature scales

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Chemistry

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  • Gas constant (R), in chemistry
  • R, one of two chiral center configurations in the R/S system
  • Alkyl group, commonly marked as R in structural formulas
  • Side chain (an unspecified group of atoms), in a chemical or structural formula

Mathematics

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Technology and engineering

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Computing

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Arts and entertainment

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Music

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Film

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Television

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Video games

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Organisations

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Transportation

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Other uses

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See also

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