By the mid-11th century the Duke of Normandy could count on more than 300 armed and mounted knights from his ecclesiastical vassals alone. By the 1020s the dukes were able to impose vassalage on the lay nobility as well.

Some duchies were bigger than kingdoms others were basically a city and the farm and forest lands around them (the Saxon duchies in the German empire or the Piast duchies of Silesia in Poland). Some duchies were sovereign states that ranked the same as a kingdom, the head of state usually called a Grand Duke.

A ducal title is widely regarded as one of the most esteemed noble ranks, second only to royalty in terms of ranking in the many social orders of Europe. The other titles that make up the noble hierarchy, such as Marquess, Count, Viscount, Baron and Lord etc, are all aristocratic titles that rank below that of a Duke.

In 1204, in the face of a French invasion, King John of England was obliged to abandon his lands in northern France. After 150 years of Anglo-Norman government, England was severed from Normandy.