Prince William and Catherine Middleton, the Duke and Duchess of Cambridge, are known by many names throughout the United Kingdom.
When William and Kate wed in 2011, Queen Elizabeth bestowed the following titles upon William: The Duke of Cambridge, Earl of Strathearn (a Scottish peerage), and Baron of Carrickfergus (an Irish peerage). Kate, therefore, acquired the titles Duchess of Cambridge, Countess of Strathearn, and Baroness of Carrickfergus.
However, following the queen’s death, it is expected that King Charles III will pass on his previous title, Prince of Wales, to his eldest son. Though it is not yet confirmed that he will do so—it’s completely up to King Charles to make that decision—it is considered highly likely that William will become the Prince of Wales, a title reserved exclusively to the heir apparent of the British throne. That would make Kate the Princess of Wales, a title that has not been held since Princess Diana passed away in 1997. (As you’ll recall, Camilla did not take the Princess of Wales title out of respect to Diana.)
There are some titles that are immediately passed on—Charles, for example, immediately became king following Queen Elizabeth’s death. Likewise, William, as the eldest son of the reigning monarch, immediately inherits the Duke of Cornwall title. And that makes Kate the new Duchess of Cornwall.