n.
Historical region of Germany, former duchy, western part of modern Hesse .
It is a thickly forested and hilly area north and east of the Rhine, crossed by the Lahn River and the Taunus Mountains. The title "Count of Nassau" was first assumed in the 12th century. Nassau joined the Confederation of the Rhine and became a duchy in 1806. It was annexed by Prussia in 1866. The present-day royal heads of The Netherlands and Luxembourg are descended from this family, called the house of Nassau.