A coregency or co-principality is the situation where a monarchical position, 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, or contemporary Andorra, where monarchical power is formally divided between two rulers.
Another example is in Ancient Egypt, mainly in the Middle Kingdom, where the Pharaoh occasionally appointed his successor 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. 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 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 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.
Lithuania
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.
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, 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.
Following the death of Tsar Feodor III of Russia in 1682, his brother Ivan and half-brother Peter were both crowned autocrats of Russia. This compromise was necessary because Ivan was unfit to rule due to physical and mental disabilities, while Peter's exclusive rule was opposed by Feodor and Ivan's older sister Sofia Alekseyevna, who led a Streltsy uprising against him and his mother's family. Because neither Tsar was of age to rule, Sofia subsequently claimed regency until she was removed from power by Peter in 1689. Ivan V and Peter I's joint reign continued, however, with Ivan maintaining formal seniority despite having little participation in the affairs of the state until his death in 1696, at which point Peter became the sole ruler.
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.
