The title “Princess Royal” was introduced to England by Charles I’s wife, Henrietta Maria, in the 17th century, a spin-off of France’s “Madame Royale” title given to the monarch’s eldest unmarried daughter. The first Princess Royal was Henrietta Maria and Charles’s daughter, Princess Mary, and it has subsequently been handed out to the monarch’s eldest daughter at their discretion. A key point of distinction with the French title, however, was that the princess’s marital status was irrelevant to her holding the title.
In 1905 King Edward VII named his eldest daughter, Louise, the Princess Royal and also moved to style her two daughters as princesses despite the fact that as daughters of a duke, they would not have otherwise. This allowed Louise’s children to have precedence immediately after other members of the British Royal Family styled as “Royal Highness.” It was an interesting move that protected the prestige of Louise in comparison to the families of her brother, the future George V, and her younger sister, Maud, who became queen consort of Norway the same year.