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).
Peerages in the United Kingdom
Individuals who use the style "Lord" or "Lady" are not necessarily peers. Children of peers use special titles called courtesy titles. The heir apparent of a duke, a marquess, or an earl generally uses his father's highest lesser peerage dignity as his own. Hence, the Duke of Devonshire's son is called Marquess of Hartington. Such an heir apparent is called a "courtesy peer", but is a commoner until such time as he inherits (unless summoned by a writ in acceleration).