Duchy (Crusader Kings 1)

Duke
A duchy is a designated area of counties that can be ruled by a Duke. As a duchy consists of counties, the said duke can rule over several Counts, gaining prestige from the more counts he has but having less of a demesne to control. Similarly, the more duchies and dukes a king has the more prestige a King gains from the number of Dukes vassal to him (or her).