Among these titles are the Duke and Duchess of Cornwall, the Duke and Duchess of Rothesay (used in Scotland), and the Earl and Countess of Chester. During engagements held alongside King Charles and Queen Camilla in Scotland last year to commemorate the Monarch's coronation, Prince William and Princess Kate utilized their Scottish titles. Notably, the title Duke of Rothesay is consistently used by the Heir Apparent, alongside other Scottish designations such as Earl of Carrick, Baron Renfrew, Lord of the Isles, and Prince and Great Steward of Scotland.
In a similar manner, while in Northern Ireland, Prince William assumes the role of Baron Carrickfergus, with Kate being addressed as Lady Carrickfergus. Carrickfergus holds historical significance as the oldest town in County Antrim, predating the capital city of Belfast in Northern Ireland.
The title of Baron Carrickfergus was bestowed upon Prince William on April 29th, 2011, as a personal gift from his grandmother, the late Queen Elizabeth II, on the occasion of his marriage to Kate. Currently, Prince William holds this title, with the Heir Apparent being his eldest son, Prince George. It's worth noting that Princess Charlotte is not in the line of succession for this particular title, as the barony is exclusively passed through the male line.