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|Basic forms of
Diarchy (or dyarchy), from the Greek δι- "twice" and αρχια, "rule", is a form of government in which two individuals, the diarchs, are the heads of state. In most diarchies, the diarchs hold their position for life and pass the responsibilities and power of the position to their children or family when they die.
Diarchy is one of the oldest forms of government. Diarchies are known from ancient Sparta, Rome, Carthage as well as from Germanic and Dacian tribes. Several ancient Polynesian societies exhibited a diarchic political structure as well. 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, with two collegial Captains Regent; and the Kingdom of Swaziland, where the joint heads of state are the king and his mother.
Current diarchies 
See Captain Regent
The Kingdom of Swaziland is a diarchy in which the king (Ngwenyama) rules in conjunction with his mother the queen mother (Indlovukazi). In practice however, most power is vested in the king, though it is often argued that the giving of authority wholesale to the royal male in this way is a neo-traditionalistic as opposed to truly traditional custom.
Former diarchies 
Roman Republic 
Swedish monarchs 
- Erik and Alrik
- Yngvi and Alf
- Björn at Hauge and Anund Uppsale
- Eric the Victorious and Olof (II) Björnsson
- Eric the Victorious and Olof Skötkonung
- Halsten Stenkilsson and Inge I
- Philip and Inge II
In England, Scotland, and Ireland 
- William III and Mary II held joint sovereignty over the three kingdoms of England, Scotland, and Ireland from 1688 to 1694.
Lithuanian monarchs 
The Lithuanian Grand Dukes typically selected submonarchs from their families or loyal subjects to assist controlling the Grand Duchy. However, the Grand Dukes remained superior.
- Vytenis (superior) and Gediminas
- Gediminas (superior) and an unknown duke of Trakai, presumably Gediminas's son.
- Algirdas (superior) and Kęstutis
- Jogaila (superior) and Kęstutis
A slightly different system developed for a brief period after Vytautas became Grand Duke, where nominally Vytautas ruled together with Jogaila, who took the title of aukščiausiasis kunigaikštis (Supreme Duke), but he has not once used the title to take any action, and in general the powers invested in the title were not clearly stated in any documents, besides the Pact of Horodlo, which guaranteed that Jogaila would have to approve the selection of a Lithuanian Grand Duke. The title was not used by any other king of Poland after Jogaila.
- Vytautas (Grand Duke) and Jogaila (Supreme Duke)
- Švitrigaila (Grand Duke) and Jogaila (Supreme Duke) for a brief period, until Švitrigaila declared war on Poland
- Sigismund I of Lithuania (Grand Duke) and Jogaila (Supreme Duke) until Jogaila's death.
Classical Sparta 
Classical Sparta in ancient Greece was ruled for over 700 years by two kings at a time (q.v., List of Kings of Sparta) who could veto one another's actions, in addition Sparta had groups of officials known as Ephors and a council of elders.
Spiritual and temporal kings 
Another common pattern of diarchy has one king in charge of spiritual matters and another, usually subordinate to the first, in charge of temporal or military matters. This pattern was followed in early Hungarian society by the spiritual kende and the military gyula. The Khazars were ruled by the spiritual khagan and the military bek. During the shogunate of Japan, the emperor held spiritual and nominal authority over the whole country, while the shogun held temporal authority.
Colonial Canada 
Other usage 
Australian Defence Organisation 
The Australian Defence Organisation (ADO) is an Australian Government organisation which consists of the Australian Defence Force (ADF) and the civilian Department of Defence personnel supporting the ADF. The Chief of the Defence Force and the Secretary of the Department of Defence jointly manage the ADO under a "diarchy" wherein both report directly to the Minister for Defence. The ADO diarchy is a governance structure unique in the Australian Commonwealth public service.
|This section may be confusing or unclear. Please help . In particular, possible confusion over recent coalitions and early twentieth century diarchy. Also who made the tour, eas the Sec of State Monatague? Chelmsford as presumable Lord Chelmsford?. (December 2009)|
A diarchy condition was introduced by Indian Council Act 1919 in Montagu-Chelmsford Report.
Introduction of diarchy in India :
On 20 August 1917 in the House of Commons, the newly appointed Secretary of State, Lord Montague made the "Grand Declaration". It said that the British policy was "increasing association of Indians in every branch of the administration and the gradual development of self-governing institutions with a view to progressive realization of responsible government in British India as an integral part of the British Empire".
In pursuance of the policy laid down in the announcements by Lord Montague, the Secretary of State and Chelmsford, the Viceroy of India made an extensive tour of India in 1917-18 and produced the Montague - Chelmsford Report containing recommendations which paved the way for Government of India Act 1919.
This act of 1919 introduced dyarchy or dual Government in the provinces. In the provinces, the executive was to be headed by a Governor who was appointed by the Secretary of State and in doing so he might consult the governor general. The Governor was responsible to the Secretary of State for acts of omission and commission. He was to maintain law and order in the province and ensure that provincial administration worked smoothly. In respect of transferred subjects, he was to be assisted by his ministers whereas reserved subjects were to be administered by the Governor and his Executive Council.
The members of the Executive council were to be appointed by Secretary of State and were responsible to him in all matters. There were certain matters which he was to administer at his own discretion. In such matters he was responsible to the Secretary of State. Each councillor was to remain in office for a period of four years. Their salaries and service conditions were not subject to the vote of provincial legislature. All decisions in the council were to be taken by a majority of votes and in case of need the Governor could use his casting vote.
Northern Ireland 
The positions First Minister and deputy First Minister operate as a diarchy and have done so since 1998. The devolved government of Northern Ireland established after the Belfast Agreement in 1998 has a system whereby the Assembly elects two leaders, one from each of the two main communities (unionist and nationalist). These two leaders actually have identical powers even though they are called First Minister and deputy First Minister respectively and serve jointly.