King of Prussia was the title Frederick the Great started using in 1772 after he took over a lot of the Kingdom of Poland.
Before this he had called himself King in Prussia.
The rulers of Berlin kept using the title after they also became German Emperors, up until Kaiser Wilhelm II abdicated (resigned) in 1918.
- ↑ Hall (trans), Colin (2005). The Electors of Brandeburg, Kings of Prussia, German Kaisers.. Karwe : Edition Rieger. ISBN 3935231644.