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
If a peer of one of the top three ranks of the peerage (a duke, marquess or earl) has more than one title, his eldest son - himself not a peer - may use one of his father's lesser titles "by courtesy". However, the father continues to be the substantive holder of the peerage title, and the son is only using the title by courtesy, unless issued a writ of acceleration. If the eldest son of a duke or marquess has an eldest son, he may use a still lower title, if one exists. In legal documents the courtesy title is implied but not used directly, e.g. the name of the person is given then "commonly called [title]".