King in Prussia
On 18 January 1701, Frederick William's son, Elector Frederick III, upgraded Prussia from a duchy to a kingdom and crowned himself King Frederick I. In the Crown Treaty of 16 November 1700, Leopold I, emperor of the Holy Roman Empire, allowed Frederick only to title himself "King in Prussia", not "King of Prussia". The state of Brandenburg-Prussia became commonly known as "Prussia", although most of its territory, in Brandenburg, Pomerania, and western Germany, lay outside Prussia proper. The Prussian state grew in splendour during the reign of Frederick I, who sponsored the arts at the expense of the treasury.