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.
Robes of the British peerage
For male peers, the Coronation robe is a cloak of crimson velvet extending to the feet, open in the front (with white silk satin ribbon ties) and trailing behind. Attached to the robe is a cape and collar of miniver pure; the rank of the peer is indicated by rows of "ermine tails (or the like)" on the miniver cape: 4 for a duke, 3½ for a marquess, 3 for an earl, 2½ for a viscount and 2 for a baron. (Royal dukes have six rows of ermine, and additional rows on the collar and on the front edges of the garment.) Peers are entitled to wear the coronation robe whether or not they are members of the House of Lords.