Full details on Prince William's titles - he is now the Duke of Cambridge, the Earl of Strathearn, and Baron Carrickfergus. After the wedding, Kate Middleton will come into them too. All titles are in the gift of the Queen and it was up to the monarch to choose which one to bestow on her grandson and his new wife.
Tradition dictates that royal men receive a title on their wedding - and often more than one. As well as a duchess, Miss Middleton will also technically be Princess William of Wales.
Here's what each of those titles mean:
Duke of Cambridge - a dukedom is the highest rank in British peerage. Dukes were the the rulers of the provinces and the King's closest peers in centuries gone by. The second duke of Cambridge, Prince Adolphus Frederick, was the seventh son of King George III. Defying the Royal Marriage Act, he married his mistress, Sarah Louisa Fairbrother, an actress and a commoner, in 1847. Since the marriage wasn't legal, his children were all illegitimate.
Earl of Strathearn - Earls are the third rank of British peerage. They were traditionally associated with shires, working in effect as local governors, but this relationship died centuries ago.
Baron Carrickfergus - Barons are the lowest rank of nobility, originally meaning someone who had pledged their loyalty to the king. Carrickfergus is a large town in Northern Ireland.