A diarchy (from Greek δι-, di-, "double",[1] and -αρχία, -arkhía, "ruled")[2][note 1] or duumvirate (from Latin duumvirātus, "the office of the two men")[4][note 2] is a form of government characterized by corule, with two people ruling a polity together either lawfully or de facto, by collusion and force. The leaders of such a system are usually known as corulers.[5]

Historically, diarchy particularly referred to the system of shared rule in British India[2] established by the Government of India Acts 1919 and 1935, which devolved some powers to local councils, which had included native Indian representation under the Indian Councils Act 1892. 'Duumvirate' principally referred to the offices of the various duumviri established by the Roman Republic.[4] Both, along with less common synonyms such as biarchy[6] and tandemocracy,[7][note 3] are now used more generally to refer to any system of joint rule or office. A monarchy temporarily controlled by two or more people is, however, usually distinguished as a coregency.

Corule is one of the oldest forms of government. Historical examples include the Pandyan dynasty of Tamilakam, Sparta's joint kingdom, the Roman Republic's consuls, Carthage's Judges, and several ancient Polynesian societies. Systems of inheritance that often led to corule in Germanic and Dacian monarchies may be included as well, as may the dual occupants of the ranks of the Inca Empire. Modern examples of diarchies are Andorra, whose princes are the President of France and the Bishop of Urgell in Catalonia; and San Marino, whose republic is led by two Captains Regent.

The status of monarchs is sometimes impugned by accusations of corule when an advisor, family member, lover, or friend

The status of monarchs is sometimes impugned by accusations of corule when an advisor, family member, lover, or friend appears to have taken too great a hand in government. Lü Buwei in Chinese history and François Leclerc du Tremblay in France are famous examples of "éminences grises" who controlled much of their countries' policies. In British history, George VI's reign was mocked as a "split-level matriarchy in pants" owing to the supposed influence of his mother, Queen Mary and his wife Queen Elizabeth.[8]

Informally shared power

Owing to Confucian notions of filial piety

Owing to Confucian notions of filial piety, Chinese and Japanese emperors were sometimes able to 'retire' but continue to exert great influence over state policy. In Indonesia, Sukarno and his vice president Mohammad Hatta were nicknamed the Duumvirate (Dwitunggal), with Sukarno setting government policy and rallying support and Hatta managing day-to-day administration. More recently, the great influence of Vladimir Putin over his successor Dmitry Medvedev was considered a duumvirate[9] or tandemocracy[10] until Putin's resumption of the office of president established him as the greater figure.[11]

Within electoral politics, governments, coalitions and parties may sometimes have two fairly equal leaders, as with: