[ Peerage ]
Duke is the highest level of peered nobility above Marquess. The female equivalent is Duchess.
The title of Duke can be kept if it was held during life, can be gained by marriage or can be granted by the Prince.
The Primogen of an upper class clan is always granted the title of Duke when they gain the position, ruling over all of the clan's Domain.
A Duke uses the Style of Lord (or Lady). The formal title is His/Her Grace the Duke of Territory, where Territory is replaced by the name of the title, traditionally the territory associated with it. The Dutch version of the formal title is “Zijne/Hare Genadigheid de Hertog(in) van Territorium”