coruler

Coruler is a term used to describe a person who shares sovereign authority with one or more other rulers in a political entity. In a coruler arrangement, power is exercised concurrently rather than by a single monarch or executive, and the specific division of duties, territory, or succession is defined by law, custom, or treaty.

Coruler arrangements can take forms such as a diarchy, co-regency, or co-emperorship. The arrangement may be codified in a constitution or in dynastic practice, and it often requires mechanisms to manage potential rivalries, clarify line of succession, and coordinate policy.

Historical instances include Sparta's diarchy, in which two kings from different royal houses ruled together and performed distinct responsibilities; the Roman Empire's late antiquity Tetrarchy created a formal multi-ruler system with two Augusti and two Caesares sharing imperial authority; and later Byzantine practice sometimes included co-emperors to secure succession. In the modern era, Andorra maintains a contemporary diarchy in which the heads of state are the president of France and the bishop of Urgell, a unique example of shared sovereignty.

Coruler can offer continuity, check rival power, and enable shared expertise, especially during transitions or in diverse polities. They can also produce ambiguity over jurisdiction, lead to conflicts if duties are not clearly defined, and complicate decision making when interests diverge.

Today the term coruler is mainly used in historical or political science contexts; related terms include co-monarch, co-regent, and joint ruler.