Royal dukedoms in the United Kingdom
In the British peerage, a royal duke is a member of the British royal family, entitled to the titular dignity of prince and the style of "His Royal Highness" who holds a dukedom. Dukedoms are the highest titles in the British roll of peerage and the holders of these particular dukedoms are Princes of the Blood Royal. The holders of the dukedoms are royal not the titles themselves. They are titles created and bestowed on legitimate sons and male-line grandsons of the British monarch, usually upon reaching their majority or marriage. The titles can be inherited but cease to be called "royal" once they pass beyond the grandsons of a monarch. As with any peerage, once the title becomes extinct, it may subsequently be recreated by the reigning monarch at any time.
Hereditary peer
A title held by someone who becomes monarch is said to "merge in the Crown" and ceases to exist, for the Sovereign cannot hold a dignity from himself. The Dukedom of Cornwall and of Rothesay, and the Earldom of Carrick, are special cases, which when not in use are said to "lapse to the Crown": they are construed as existing, but held by no one, during such periods. These peerages are also special because they are never directly inherited. The Dukedom of Cornwall was held formerly by the eldest son of the King of England, and the Dukedom of Rothesay, the Earldom of Carrick, and certain non-peerage titles (Baron of Renfrew, Lord of the Isles and Prince and Great Steward of Scotland) by the eldest son of the King of Scotland. Since those titles have been united, the dukedoms and associated subsidiary titles are held by the eldest son of the monarch. In Scotland, the title "Duke of Rothesay" is used for life. In England and Northern Ireland, the title "Duke of Cornwall" is used until the heir-apparent is created Prince of Wales. At the same time as the Principality is created, the Duke is also created Earl of Chester. The earldom is a special case, because it is not hereditary, instead revesting or merging in the Crown if the Prince succeeds to the Crown or predeceases the monarch: thus George III was created Prince of Wales and Earl of Chester a month after his father's death.