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
Any woman who marries a peer uses the feminine version of his peerage title, even if her own precedence is higher than his, as in the case of a duke's daughter marrying a baron, because a peerage is a substantive title, the usage of which is preferred to any courtesy style - unless she marries into the Royal Family. If a woman marries the younger son of a duke or marquess, she becomes "Lady ["Husband's given name"] ["Husband's surname"]." If she marries the younger son of an earl, viscount or baron, she becomes "The Hon. Mrs ["Husband's given name"] ["Husband's surname"]."