Peerage of England
The ranks of the English peerage are, in descending order, Duke, Marquess, Earl, Viscount, and Baron. While most newer English peerages descend only in the male line, many of the older ones (particularly older baronies) can descend through females. Under English inheritance law all daughters are co-heirs, so many older English peerage titles have fallen into abeyance between various female co-heirs.
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"]."