Dieu et mon droit (French for God and my [birth] right) has generally been used as the motto of the British monarch since it was first adopted by Henry VI in the early 15th century. It's also found on the Monarch's Coat of Arms
In choosing a motto in French rather than English, it should be noted that the English language had only recently replaced French as the language of the English ruling classes - and Henry spoke French and had been crowned King of France as well as of England. The motto of the Order of the Garter Honi soit qui mal y pense is also in French.
The phrase was allegedly first used as a password by King Richard I in 1198.
Dieu et mon droit was the inspiration for The Beatles joke motto Duit On Mon Dei, later adopted as an album title by Harry Nilsson.
