A duchy is a territory or domain ruled by a duke or duchess. Historically, some duchies in Continental Europe were sovereign, while others (especially in France and Britain) were subordinate districts of a kingdom.
Traditionally, a grand duchy, such as Luxembourg, was generally independent and sovereign. Sovereign duchies were common in the Holy Roman Empire and German-speaking areas. In France, a number of duchies existed in the medieval period. Queen Elizabeth II of the United Kingdom still holds the medieval French title of Duke of Normandy; the only lands still attached to the Duchy of Normandy are the Channel Islands.
Today, duchies aren't very common in Europe, primarily because a lot of former monarchies such as France and Russia had their revolutions. However, duchies will always be remembered - in our history textbooks.
- The Duchy of Cornwall - On the Prince of Wales' official web-site
- Tyr Gwyr Gweryn The Duchy charters, Cornish foreshore case and much more in full