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.
History of the British peerage
At first, earls and barons were the only ranks in the peerage. The other ranks of the Peerage developed in the fourteenth and fifteenth century. In 1337, Edward, the Black Prince was created Duke of Cornwall, taking precedence over all earls. Dukedoms were reserved for members of the Royal Family until 1387, when Robert de Vere, 9th Earl of Oxford, the favourite of Richard II, was created Duke of Ireland for life. De Vere had previously been created Marquess of Dublin for life, making him the first person to hold a dignity of such a rank between Dukes and Earls. Subsequent marquessates were created rarely; the Marquess of Winchester, whose dignity was created in 1551, is the only English marquess without a dukedom. The rank of viscount was introduced from Europe in 1440, when John, Baron Beaumont, was created Viscount Beaumont, with precedence between earls and barons.