When Prince William eventually becomes King the role will bring with it new titles for other members of his family.
While sons Prince George and Prince Louis are likely to receive dukedoms when they get married, a royal rule means their sister Charlotte will not automatically become a Duchess on hers.
Princess Charlotte, whose current full title is Her Royal Highness Princess Charlotte of Cambridge, can only become a Duchess if she marries a Duke.
Instead, as the only daughter of the Duke and Duchess of Cambridge, she is likely to earn the title of Princess Royal once William becomes King.
This title traditionally goes to the eldest daughter of a sitting monarch – but it is not automatic.
It must be bestowed upon them by the monarch and only as long as the title is free.
The title of Princess Royal has been held by the Queen’s only daughter, Princess Anne, since 1987.
She did not become Princess Royal straight after the death of her predecessor Princess Mary, but had to wait for over 20 years of the title being vacant until it was bestowed upon her.
Princess Royal has a lifelong tenure, meaning Princess Anne will hold it until her death.
It is expected that William will make Charlotte the Princess Royal once the title is vacant, and he is King.
This title has been awarded to the monarch’s eldest daughter for centuries, beginning in 1642 with Princess Mary, the daughter of King Charles I.
While it is purely ceremonious, under ancient British law, any man who sleeps with the Princess Royal before marriage is guilty of high treason.