Dukes in the United Kingdom
Duke, in the United Kingdom, is the highest-ranking hereditary title in all four peerages of the British Isles. A duke thus outranks all other holders of titles of nobility (marquess, earl, viscount and baron).
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"]."