Duchy of Lancaster
The Duchy of Lancaster derives from an earldom created by Henry III for his younger son, Edmund, and enlarged by many of the estates of Simon de Montfort after his death at Evesham. Edmund's grandson, Henry Grosmont, a friend and supporter of Edward III, was created a duke in 1351, and the duchy with its vast estates and income came to John of Gaunt through his marriage to Henry's only child, Blanche.
The title reverted to the Crown when John of Gaunt's son, Henry (Shakespeare’s Bolingbroke) usurped the throne of Richard II in 1399 and ruled as Henry IV. He, his son, Henry V and grandson, Henry VI make up the royal House of Lancaster, and the term “Lancastrian” refers to these kings, their retainers and supporters; and all those that fought with the Lancastrians or Henry Tudor against the Yorkists in the Wars of the Roses.
The duchy remains the preserve of the monarch to this day, with one of the less onerous ministerial positions being the “Chancellor of the Duchy of Lancaster”.