diarchy

Diarchy is a form of government in which two individuals hold the sovereign authority and exercise executive power jointly. In a diarchy the two rulers typically share equal status, with powers defined by a constitution or legal framework; the arrangement may involve fixed terms, rotating offices, or a deliberate division of competences to avoid unilateral rule. The term is used in political science to describe both historical and contemporary instances, and is distinct from a duumvirate, which often denotes a temporary or informal joint rule.

Historical examples include ancient Sparta, where two kings from different dynastic lines ruled together, and the Roman Republic's system of two consuls who jointly governed for a single year. In these cases the arrangement functioned as a check on monarchical power and provided stability through shared leadership, though real power could be asymmetric in practice.

Contemporary or constitutional diarchies include Andorra, which has two co-princes—the president of France and the bishop of Urgell—whose roles are largely ceremonial with executive power in a separate government. San Marino maintains a diarchic head of state through annually elected captains regent. Other cases may occur in federal or disputed contexts, or as symbolic power-sharing arrangements.

Advantages of diarchy can include representation of diverse groups, checks and balance, and reduced risk of tyranny; drawbacks include potential deadlock, diluted accountability, and inefficiency if the two rulers disagree. Successful diarchies typically rely on clear constitutional rules, defined power boundaries, and dispute-resolution mechanisms.