The royal privilege that Kate must not use until William becomes King

When they married in 2011, Prince William and Kate Middleton received a very special gift from the Queen; William was given the dukedom of Cambridge, thus making Kate a Duchess.

While she is technically listed as a Princess of the United Kingdom on her children's birth certificates, Kate goes by the Her Royal Highness the Duchess of Cambridge title. Although she is often referred to as Kate or Catherine, her official HRH title does not actually include her first name, and it won't for many years to come.

Royal expert Marlene Koenig explained why, telling Business Insider: 'If the Queen had not created William a duke, Catherine would be styled as HRH Princess William of Wales.'

She explained that Kate will only be officially allowed to use her first name when William becomes the King and she steps into her role as Queen consort. Then, according to Koenig, she will be known as Queen Catherine.

It is royal tradition for women to take their husband's titles when they tie the knot, with Kate becoming a Duchess when the Queen awarded William with the Duke of Cambridge title.

However, when men marry they do not adopt the titles of royal women which is why Prince Philip was made Duke of Edinburgh before he married the Queen, who was Princess Elizabeth at the time. It also explains why Princess Beatrice and Princess Eugenie's husbands didn't receive Prince titles when they married into the royal family.

When William does take the throne and Kate becomes Queen Consort, it is expected that he will hand his dukedom down to Prince George or Prince Louis.