In Anglo-Saxon England (the time before the Norman Conquest of 1066) the highest rank beneath that of the king was an earl. The English title of duke was created by the Plantagenets when Edward III made his eldest son, Edward, the Black Prince, the 1st Duke of Cornwall in 1337. The Duke of Cornwall remains today the highest-ranking duke in England. The title, as it has been since its creation, is traditionally held by the monarch’s eldest son.
The premier non-royal duke in England is the Duke of Norfolk, a title created in 1483 for Sir John Howard (the Howard dukes of Norfolk also have the titles of earl marshal and hereditary marshal of England). Confusingly, this was actually the third creation of the title. The first Dukedom of Norfolk was created for Thomas de Mowbray in 1397 and became extinct in 1476, with the passing of his great-grandson, the 4th Duke, who died without male heirs. The title was created for the second time in 1481 for Richard of Shrewsbury, Duke of York, the second son of Edward IV. One of the famous Princes in the Tower, the Duke of York’s titles were declared forfeit by his uncle, Richard III, when he seized the crown and declared the Duke of York (and his brother, Edward V) illegitimate.
A duke’s wife is a duchess; both are are addressed as “your grace,” or simply “duke” or “duchess.” A duke and duchess are the only British peers not called “lord” or “lady.”
A British duke wears a coronet (a gold-plated metal circlet with faux jewels pressed from the metal) with eight strawberry leaves (see above). The coronet is traditionally worn only at coronations.
Image of a duke’s coronet by Sodacan / Wikipedia. Image licensed under the Creative Commons Attribution-Share Alike 3.0 Unported, 2.5 Generic, 2.0 Generic, and 1.0 Generic licenses.