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
Titles with the same name as a peer's main title are also not used as courtesy titles. For instance, The Duke of Westminster is also The Marquess of Westminster and The Earl Grosvenor (amongst other titles). The duke's eldest son is not styled "Marquess of Westminster" (which would cause confusion between the son and the father), and so is styled "Earl Grosvenor" instead. The title used does not have to be exactly equivalent to the actual peerage: the eldest son of the current Duke of Wellington is styled Marquess "of" Douro, although the actual peerage possessed by his father is Marquess Douro (not "of" Douro).