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.
Courtesy titles in the United Kingdom
If a peer of one of the top three ranks of the peerage (a duke, marquess or earl) has more than one title, his eldest son - himself not a peer - may use one of his father's lesser titles "by courtesy". However, the father continues to be the substantive holder of the peerage title, and the son is only using the title by courtesy, unless issued a writ of acceleration. If the eldest son of a duke or marquess has an eldest son, he may use a still lower title, if one exists. In legal documents the courtesy title is implied but not used directly, e.g. the name of the person is given then "commonly called [title]".