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. It also denotes a king's imperial status through the acquisition of an empire or vice versa. 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" instead of "emperor-king".
The British Crown had officially taken over the governing of British India from the East India Company in 1858, in the aftermath of what the British called 'the Indian Mutiny'. Henceforth, the new British Raj was ruled directly from Whitehall via the India Office. Following the Delhi Durbar in 1877, Queen Victoria was given Imperial status by the British Government, and she assumed the title Empress of India. She was thus the Queen-Empress, and her successors, until George VI, were known as King-Emperors. This title was the shortened form of the full title, and in widespread popular use. The reigning King-Emperors or Queen-Empress used the initials R I or the abbreviation Ind. Imp. after their name. British coins, and those of the British Empire and Commonwealth dominions, routinely included some variation of the titles Rex Ind. Imp., although in India itself the coins said "Empress", and later "King Emperor." When, in August 1947, India became independent, all dies had to be changed to remove the latter two abbreviations, in some cases taking up to a year. In Great Britain, coins of George VI carried the title up to 1948.
The Serbian emperor Stefan Dušan, earlier king, is attested with the title "Emperor of Greece and King of All Serb Lands and the Maritime" in a document dating to between 1347–56. Dušan has been described as a "king-emperor".
John VI of Portugal was made titular Emperor of Brazil alongside being King of Portugal and was titled as King-Emperor until his death. After John VI's death, his son Pedro briefly acceded him as King of Portugal while reigning as Emperor of Brazil.