It is to be distinguished from diarchies or duumvirates (such as ancient Sparta and Rome), and also from regencies where a single monarch (usually infant or incapacitated) has a seniority above a regent but exercises no power except legally holding the position of head of state.
Co-principality is a distinct but related system employed in contemporary Andorra, where monarchical power is formally divided between two rulers.
[1] 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.
Yet, exceptions can be found, 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.
Then, latest in year 7[5] of Thutmose III's reign, she took over royal regalia and was then titled King of Egypt under the Throne name (prenomen) Maatkare.
Henry the Young King was not permitted to exercise royal authority and his title as co-king was effectively a sinecure to denote his status as his father's chosen heir.
The Monarchy of England experienced joint rule under the terms of the act sanctioning the marriage of Mary I to Philip II of Spain.
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.
Co-regency was common amongst the Chinese ethnics both formally or informally dating far back to the Han dynasty to the Late Qing dynasty, if it happened to be so that a new Emperor ascended the throne and was mature but not well adept with politics, it was a national concern for stability in which the monarch was the head of state de jure whereas a co-regent was its de facto ruler.
It was common during the Principate for a Roman emperor or Augustus to appoint Caesar as designated heir and junior co-emperor, in many cases adopting them as their son, who did not necessarily have to be biologically related to them.