It's complicated.The Queen has stipulated that all her male descendants who do not bear the titular dignity of Prince shall use Mountbatten-Windsor as their surname.Willam and Harry, though, DO "bear the titular dignity of Prince".For these cases, children generally use "Prince X of Y" where X is their given name and Y is their father's territorial designation. For William and Harry, their father is Prince Charles, who is the Prince of Wales, so they were "Prince William of Wales" and "Prince Harry of Wales".This is normally dropped when such children reach adulthood and simply use their title. You need a last name when you might get confused with someone else. The likelihood of, say, Prince William, Duke of Cambridge getting confused with some other Prince William, Duke of Cambridge is pr