The Duchy of Lancaster was created for John of Gaunt, a son of King Edward III, in the year 1362. When he died in 1399 the Duchy passed to his son, Henry Bolingbroke..
Boling…broke usurped the throne from King Richard II the same year and became King Henry IV..
As king, he merged the Dukedom with the Crown and decreed that it would be personally inherited by the monarch..
The present Duke of Lancaster is Her Majesty Queen Elizabeth II - and yes, she is The Duke, not the Duchess. (MORE)