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
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).