A monarch is the person who heas a monarchy, a form of government in which the country or entity usually ruled or controlled by an individual who usually rules for life or until abdication. Monarchs may be autocrats (absolute monarchy) or may be ceremonial heads of state who exercise little or no power or only reserve power, with actually authority vested in a parliament or other body (constitutional monarchy). Most states only have a single monarch at any given time, although a regent may rule when the monarch is a minor, not present or debilitated. Two monarchs have ruled simultaneously in some countries, as in the ancient Greek city-state of Sparta or the joint sovereignty of spouses or relatives (eg. William and Mary of Kingdom of England and Scotland, Peter and Ivan of Russia, Charles and Joanna of Castile, etc).