A king-emperor, the female equivalent being queen-empress, is a sovereign ruler who is simultaneously a king of one territory and emperor of another. This title usually results from a merger of a royal and imperial crown, but recognises that the two territories are different politically or culturally and in status (emperor being a higher rank than king). It also denotes a king's imperial status through the acquisition of an empire or vice versa.
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The dual title signifies a sovereign's dual role, but may also be created to improve a ruler's prestige. Both cases, however, show that the merging of rule was not simply a case of annexation where one state is swallowed by another, but rather of unification and almost equal status, though in the case of the British monarchy the suggestion that an emperor is higher in rank than a king was avoided by creating the title "king-emperor" ("queen-empress") instead of "emperor-king" ("empress-queen").