A duchy is a territory or domain ruled by a duke or duchess. It is often part of a larger kingdom or empire and can vary in size and governance. Historically, duchies were significant in medieval Europe, where they played a crucial role in the feudal system, with nobles managing land and resources. Duchies can still exist today, although their political power is usually limited. Some well-known examples include the Duchy of Cornwall in the United Kingdom, which provides income to the heir apparent, and the Duchy of Luxembourg, a sovereign state ruled by a grand duke.