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 along with a regency where the monarch in most cases infant or incapacitated particularly has a senior above him and exercises no power whatsoever apart from legally holding the position of Head of State. Andorra 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