Prince Williams full name is William Arthur Philip Louis Mountbatten-Windsor. However, members of the British Royal Family did not have surnames before 1917, and instead used the name of the dynasty or house they belonged to. The surname Mountbatten-Windsor first appeared on an official document in 1973, and it was declared in the Privy Council that The Queens descendants, other than those with the style of Royal Highness and the title of Prince/Princess, or female descendants who marry, would carry the name of Mountbatten-Windsor. Therefore, all of The Queens children, including Prince William, have the surname Mountbatten-Windsor. However, members of the Royal Family who are entitled to the style and dignity of HRH Prince or Princess do not need a surname, but if at any time any of them do need a surname (such as upon marriage), that surname is Mountbatten-Windsor.