Diarchy (or dyarchy, meaning "I rule") is a form of government in which two individuals ("diarchs") are joint heads of state. Most diarchs hold their position for life, passing the position to their children or other family members.
Diarchy is one of the oldest forms of government: examples include ancient Sparta, Rome, Carthage as well Germanic and Dacian tribes. Several ancient Polynesian societies also exhibited a diarchic political structure. Ranks in the Inca Empire were structured in moieties, with two occupants of each rank, but with different prestige, one hanan (upper) and one hurin (lower). In modern usage, diarchy means a system of dual rule, whether this be of a government or of an organization. Such 'diarchies' are not hereditary.
Modern examples of diarchies are the Principality of Andorra, whose heads of state are the President of France and the Bishop of Urgell; the Republic of San Marino, led by two collegial Captains Regent; and the Kingdom of Swaziland, where the joint heads of state are the King and his mother.

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