Overview
Princess Royal is a style customarily (but not automatically) awarded by British monarchs to their eldest daughter. Although purely honorary, it is the highest honour that may be given to a female member of the royal family. There have been seven Princesses Royal. Princess Anne became Princess Royal in 1987.
The style Princess Royal came into existence when Queen Henrietta Maria (1609–1669), daughter of Henry IV, King of France, and wife of King Charles I (1600–1649), wanted to imitate the way the eldest daughter of the King of France was styled "Madame Royale". Thus Princess Mary (born 1631), the daughter of Henrietta Maria and Charles, became the first Princess Royal in 1642.
It has become established that the style belongs to no one by right, but is given entirely at the sovereign's discretion. Princess Mary (later Queen Mary II) (1662–1694), the eldest daughter of King James II, and Princess Sophia Dorothea (1687–1757), the only daughter of King George I, were eligible for this honour but did not receive it. At the time they respectively became eligible for the style, Princess Mary was already Princess of Orange, and Sophia Dorothea was already Queen in Prussia. A Princess Royal has never acceded to the British throne, though Princess Victoria, the eldest daughter of Queen Victoria, was simultaneously Princess Royal and heir presumptive for a year until she was displaced by the birth of her brother Prince Albert Edward.
Princess Louisa Maria (1692–1712), the youngest daughter of King James II (died 1701), born after he lost his crown in the Glorious Revolution of 1688–1689, was considered to be Princess Royal during James's exile by Jacobites at Saint-Germain-en-Laye and was so called by them, even though she was not James's eldest living daughter at any time during her life.
From Wikipedia (CC BY-SA 4.0).