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).
British nobility
All modern British honours, including peerage dignities, are created directly by the British monarch, taking effect when letters patent are affixed with the Great Seal of the Realm. The Sovereign is considered the fount of honour and, as "the fountain and source of all dignities cannot hold a dignity from himself", cannot hold a British peerage.