R. or r. may refer to:

• Reign, the period of time during which an Emperor, king, queen, etc., is ruler.
• Rex, abbreviated as R., the Latin word meaning King
• Regina, abbreviated as R., the Latin word meaning Queen
• Regnavit or rexit, abbreviated as r., used in historiography to designate the ruling period of a person in dynastic power, to distinguish from his or her lifespan (e.g. "Charles V (r. 1519–1556)")
• Abbreviation R., meaning Rex or Regina. Commonly used for criminal prosecution in Commonwealth realms where offences are brought in the name of the monarch (e.g. "R. v Defendant")
• R., an album by American singer R. Kelly

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• R (disambiguation)
• All pages with titles beginning with R.
• All pages with titles containing R.
• R, eighteenth letter of the English and Latin alphabets

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