A duchy is a territory, fief, or domain ruled by a duke or duchess.

Some duchies were sovereign in areas that would become unified realms only during the Modern era (such as Germany and Italy). In contrast, others were subordinate districts of those kingdoms that unified either partially or completely during the Medieval era (such as England, France, and Spain). The term is used almost exclusively about Europe.

For the history of duchies as an institution, see Duke.

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