LONDON, ENGLAND - JUNE 11: Catherine, the Duchess of Cambridge (C), joins members of the royal family to celebrate the Queen's official birthday by taking part in the Trooping the Colour parade on June 11, 2011 in London, England. The ceremony of Trooping the Colour is believed to have first been performed during the reign of King Charles II. In 1748, it was decided that the parade would be used to mark the official birthday of the Sovereign. More than 600 guardsmen and cavalry make up the parade, a celebration of the Sovereign's official birthday, although the Queen's actual birthday is on 21 April. (Photo by Oli Scarff/Getty Images)