Most monarchies only have a single person acting as monarch at any given time, although two monarchs have ruled simultaneously in some countries, a situation known as diarchy. Historically this was the case in the ancient Greek city-state of Sparta. There are examples of joint sovereignty of spouses, parent and child or other relatives (such as William III and Mary II in the kingdoms of England and Scotland, tsars Peter I and Ivan V of Russia, and Charles I and Joanna of Castile).