King in Prussia
By the act of coronation in Königsberg Castle on 18 January 1701, Frederick William's son, Elector Frederick III, became Frederick I, King in Prussia. The elevation of the Duchy of Prussia to the Kingdom of Prussia was possible because the Hohenzollerns' authority in Prussia was independent of Poland and the Holy Roman Empire. Since "Kingdom of Prussia" was increasingly used to designate all of the Hohenzollern lands, former ducal Prussia became known as the Province of Prussia (1701–1773), with Königsberg as its capital. However, Berlin and Potsdam in Brandenburg were the main residences of the Prussian kings.