Kate’s husband is the eldest son of the heir apparent meaning he will one day become king.
Upon Prince Charles’ accession to the throne, Prince William will become the heir apparent and will be made the next Prince of Wales.
Until that time, however, Prince William holds the titles of Duke of Cambridge, Earl of Strathearn and Baron Carrickfergus.
This means Kate uses the titles of the Duchess of Cambridge, Countess of Strathearn and Lady Carrickfergus.
