Coregency
A coregency is the situation where a monarchical position (such as prince, princess, 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. Co-principality is a distinct but related system employed in contemporary Andorra, where monarchical power is formally divided between two rulers. Historical examples. Coregencies were common in the Hellenistic period; according to one scholar, they "can usually be explained as a means of avoiding crises of succession or internal conflict, and of strengthening  dynastic identity and ideology." Other examples 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. Ancient Egypt. In Ancient Egypt, coregency was quite problematic as the Pharaoh was seen as the incarnation/representation of the god Horus. Therefore, according to the divine order Ma'at, only  one King could exist at the same time. Yet, exceptions can be found., mainly in the Middle Kingdom, where the occasionally appointed his successor (often one of his sons) as coregent, or joint king, to ensure a smooth succession. “This system was used, from at least as early as the Middle Kingdom, in order to ensure that the transfer of power took place with the minimum of disruption and instability”. Coregencies are highly probable for Amenemhat I > Senusret I > Amenemhat II > Senusret II. Most probably the real king in power was the older one (father) adopting the  younger ruler (son), while the co-regent had to wait until after the death of the older one to really have access to full royal power. Yet, the years of reigns normally were counted from the beginning of the coregency on. Due to this and to the fragmentary character of known sources, the establishment of Egyptian chronology was quite complicated and remains disputed up to date. Yet, understanding the existence of co-regency reduced the chaos quite a lot. The institution of coregency is different from that of vice-regency, where an adult person (in Ancient Egypt often the mother  of the king) functions as Legal guardian, ruling in the name of the underage king. Some of the female vice-regents of Egypt rose to a status of equal to the God-Kings, becoming co-rulers as can be seen in the famous case of Hatshepsut. After the death of her husband Thutmose II, Hatshepsut ruled in the name of Thutmose III, her nephew and stepson. Then, latest in year 7 of Thutmose III's reign, she took over royal regalia and was then titled King of Egypt under the Throne name ("prenomen") Maatkare. For later periods of Pharaonic Egyptian  history, the existence of the institution of coregency has been put into question by Egyptologists, while, "the Ptolemaic and Roman period examples being the most securely identified". Ancient Israel. 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. Britain. 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. China. 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). 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 "" (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. Russia. 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. Sweden. The monarchy in 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. Co-principality. The only co-principality in the world is Andorra, a small south-western European country located on the Iberian Peninsula. As part of a peace agreement, joint rule between the Count of Foix and the Bishop of Urgell was established by a formal act of Paréage in 1278. The counts' right eventually passed to the French state, and joint rule between France (now embodied by its president) and the Bishop of Urgell was codified in the Constitution of Andorra in 1993.