There's more: The palace statement said William was also named the earl of Strathearn and baron Carrickfergus. Middleton will take those titles as well, becoming countess of Strathearn and baroness Carrickfergus when she says: "I will."
Strathearn ties William and Middleton to Scotland, where the pair met and fell in love. Baron Carrickfergus is a little-used title which refers to one of the oldest towns in Northern Ireland.
All three titles were bestowed by William's grandmother, Queen Elizabeth II, to mark the prince's marriage and were announced early Friday via Twitter, by email, and on the royal wedding's official website.
Because Middleton was not born a princess, she will not officially become Princess Catherine -- although the public may choose to call her that, or even "Princess Kate," in defiance of protocol.
Royal watchers called the bestowal of the title duke of Cambridge a personal mark of esteem from the queen. It refers to the history-steeped university town that is a symbol of British prestige. The dukedom's history stretches back to Medieval times and has for 300 years been associated with royalty.
Jennie Bond, one of the U.K.'s foremost experts on the monarchy -- and a paid royal wedding consultant for The Associated Press -- said there was a hint that William was going to get the title. "The queen went to visit Cambridge the day before yesterday so a lot of people thought that was how it was going to be," she said.
Bond called the title "a personal gift from the queen, a mark of her esteem for her grandson."
Barons, viscounts, earls, marquesses and dukes are all orders of British nobility, in ascending order of prestige. The titles can be created and become extinct, for example when a duke or earl ascends to the throne or when he dies without leaving legitimate heirs.
In 1706, George Augustus -- who subsequently became King George II -- was made the duke of Cambridge. The dukedom ceased when he ascended to the throne in 1727, but was recreated in 1801.
Although a venerable title, it does not necessarily have terribly pleasant history.
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, and the dukedom became extinct on his death, in 1904.