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).
The wife of a duke is known as a duchess, which is also the title of a woman who holds a dukedom in her own right, referred to as a duchess "suo jure"; her husband, however, does not receive any title. In the order of precedence in the United Kingdom, non-royal dukes without state offices or positions generally take precedence before all other nobility, in order of date of creation, but after royalty and certain officers of state.
Royal dukedoms.
A royal duke is a duke who is a member of the British royal family, entitled to the style of "His Royal Highness". The current royal dukedoms are, in order of precedence of their holders (that is, not in order of precedence of the dukedoms themselves): 
With the exceptions of the dukedoms of Cornwall and Rothesay (which can only be held by the eldest son of the Sovereign), royal dukedoms are hereditary, according to the terms of the letters patent that created them, which usually contain the standard remainder to the "heirs male of his body". The British monarch also holds and is entitled to the revenues of the Duchy of Lancaster, and within the borders of the County Palatine of Lancashire is by tradition saluted as "The Duke of Lancaster". While in the Channel Islands, the monarch is The Duke of Normandy. Even when the monarch is a Queen regnant, she does not use the title of "Duchess".
Non-royal dukedoms.
These are extant non-royal dukes in the United Kingdom.
Coronet.
A British or Irish duke is entitled to a coronet (a silver-gilt circlet, chased as jewelled but not actually gemmed) bearing eight conventional strawberry leaves on the rim of the circlet. The physical coronet is worn only at coronations. Any peer can bear his coronet of rank on his coat of arms above the shield.