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).
List of viscountcies in the peerages of Britain and Ireland
The majority of viscountcies are held by peers with higher titles, such as duke, marquess or earl; this can come about for a number of reasons, including the title being created as a subsidiary title at the same time as the higher peerage, the holder being elevated at a later time to a higher peerage or through inheritance when one individual is the heir to two separate titles.