At that time, the Duchy of Prussia was only the area east of the place where the Vistula River enters the sea. In 1618, the new Duke of Prussia was the Elector John Sigismund of Brandenburg. He was also Margrave of Brandenburg. Brandenburg was ruled by the Hohenzollern family. The Duchy of Prussia was important to the Hohenzollern family because it was not part of the Holy Roman Empire. The name for the new state was "Brandenburg-Prussia". In the middle of the state was Polish land, but Brandenburg-Prussia was moving away from Poland. Under Frederick William, who was called "the Great Elector," Prussia took some new land in Magdeburg and areas west of the Rhine.