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.
List of viscountcies in the peerages of Britain and Ireland
The majority of viscountcies are held by peers with higher titles, such as duke, marquess or earl; this can come about for a number of reasons, including the title being created as a subsidiary title at the same time as the higher peerage, the holder being elevated at a later time to a higher peerage or through inheritance when one individual is the heir to two separate titles.