What is the title for the ruler of a duchy?

The story behind the answer

In the medieval feudal hierarchy, a duchy was governed by a ducal ruler (duke or duchess), ranking below kings and sometimes princes. By contrast, a minister was an appointed administrator, not a territorial lord; a marquis (or marquess) oversaw a march, a fortified border region; and a count managed a county (earl in England).

Duchies such as Burgundy, Normandy, and Bavaria could be powerful and semi-autonomous, with their own courts, coinage, and vassals, often commanding multiple counties. These titles reveal the layered, decentralized nature of medieval politics, where authority was distributed through a web of oaths and landed jurisdictions rather than a single centralized state.