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.
Peerages in the United Kingdom
Individuals who use the style "Lord" or "Lady" are not necessarily peers. Children of peers use special titles called courtesy titles. The heir apparent of a duke, a marquess, or an earl generally uses his father's highest lesser peerage dignity as his own. Hence, the Duke of Devonshire's son is called Marquess of Hartington. Such an heir apparent is called a "courtesy peer", but is a commoner until such time as he inherits (unless summoned by a writ in acceleration).