The title, Duke, (from Latin Dux, a leader) is the highest in the British Peerage. As there are no British "princes" outside the blood-royal, so pre-eminent in dignity is the ducal title that each royal prince, shortly after attaining his majority (age 21) is usually, but not always, created a Duke; the titular style of Prince, apart from the Prince of Wales, is a title of courtesy. Thus, Prince Henry, son of King George V, was created Duke of Gloucester, Prince Andrew, son of Queen Elizabeth II, was created Duke of York.
Since the title Duke signified Sovereign status (William the Conqueror was Duke of Normandy) it was not adopted until 1337, when Edward III conferred the Dukedom of Cornwall on his eldest son, the Black Prince. This was followed by Henry Duke of Lancaster in 1351. The first subject to receive a dukedom who was not a member of the royal family, nor one nearly connected, was Sir William de la Pole, Marquess of Suffolk, who was created Duke of Suffolk in 1448.
A Duke is styled Most Noble (or less formally His Grace), and by the Sovereign in public instruments, Our right trusty and right entirely beloved cousin, with the addition of and counselor when a member of the Privy Council.