The phrase William and Mary usually refers to the joint sovereignty over the
Kingdom of England, as well as the
Kingdom of Scotland, of
King William III and his wife
Queen Mary II, a daughter of James II. Their joint reign began in February, 1689, when they were called to the throne by Parliament, replacing
James II, who was "deemed to have fled" the country in the
Glorious Revolution of 1688. After Mary died in 1694, William of Orange ruled alone until his death in 1702. Their rule was the only period in British history in which "joint sovereigns" with equal powers were allowed to reign; usually, the spouse of the monarch has no power and is simply a
consort.
See more at Wikipedia.org...