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A coregency or co-principality is the situation where a monarchical position (such as king, queen, emperor or empress), normally held by only a single person, is held by two or more. It is to be distinguished from diarchies or duumvirates such as ancient Sparta and Rome where monarchal power is formally divided between two rulers.
Historical examples of this include the coregency of Frederick I of Austria and Louis the Bavarian over the Kingdom of Germany. Jure uxoris Kings in Kingdoms such as Spain and Portugal can also be found (Ferdinand V and Isabella I of Castile, Philip I and Joanna of Castile, Peter III and Maria I of Portugal, etc.). In Navarre, the husbands of queens regnant were styled as co-rulers.
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Another example is in Ancient Egypt, mainly in the Middle Kingdom, where the Pharaoh occasionally appointed his successor (often one of his sons) as coregent, or joint king, to ensure a smooth succession. The Pharaoh also did this when he was elderly or unable to rule his country on his own (such as the case of Thutmose III and Amenhotep II or Amenemhat I and Senusret I). The existence of the practice makes establishing firm dates in Egyptian chronology more of a challenge, as the lengths of coregencies are often uncertain and complicate the use of accepted regnal lengths to establish dates. Some of the Queens of Egypt rose to a status of equal to the God-Kings, becoming co-rulers and / or at least as important in religious affairs, and were even portrayed with the same size as their male consort and even with the same size as the other Gods of Egypt. Such were the cases of Hatshepsut, Nefertiti, Nefertari and the Nubian Egyptian Queens. In the Ptolemaic Dynasty women finally rose to become equal co-rulers with men and even challenging them in order to become their respective consorts. This was due to a progressive improvement of the already high status of women in the Egyptian society, as well as to the religious principle of balance (Maat) between male and female. In Nubia, Queens like Amanishakheto and Amanitore were crowned alongside Kings at Dangeil and had both their pyramids at Meroë with the same height and side by side, and exercised power at the same level, even commanding armies. In Ethiopia, Kandakes also reached and hold this or a similar status.
The Gonghe Regency (meaning joint harmony) of the Zhou dynasty in China was ruled jointly by two dukes for a short period according to Han Dynasty historian Sima Qian, but it is more likely that the Count of Gong was the actual single ruler (according to bronze tapestries).
The monarchy of Sweden has had several periods of joint rule: Erik and Alrik, Yngvi and Alf, Björn at Hauge and Anund Uppsale, Eric the Victorious and Olof Björnsson, Eric the Victorious and Olof Skötkonung, Halsten Stenkilsson and Inge I, and Philip and Inge II.
The monarchy of England experienced joint rule under the terms of the act sanctioning the marriage of Mary I to Philip II of Spain. Philip notionally reigned as king of England (inclusive of Wales) and Ireland by right of his wife from 1554 to 1558. Similarly, following the Glorious Revolution, Mary II and her husband William III held joint sovereignty over the kingdoms of England, Scotland, and Ireland from 1688 to 1694.
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.
In the book The Mysterious Numbers of the Hebrew Kings, Edwin R. Thiele proposed co-regency as a possible explanation for discrepancies in the dates given in the Hebrew Bible for the reigns of the kings of Israel and Judah. At least one co-regency is explicitly documented in the Bible: the coronation of King Solomon occurred before the death of his father David. Some Kings of Egypt, especially during the Twelfth Dynasty, also practiced this custom by associating their own sons in order to both prepare them for the office and prevent anyone else from usurping the throne.