Reign, the period of time during which an Emperor, king, queen, etc., is ruler
Rex or regina, abbreviated as R., the Latin words meaning "King" and "Queen", respectively
Commonly used for criminal prosecution in Commonwealth realms where offences are brought in the name of the monarch (e.g. "R. v Defendant")
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)")
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