It is the little word in that makes the title "King in Prussia" an extraordinary one.

It is the title of the Prussian kings from 1701 until 1740. They resided in their electorate Brandenburg (Berlin, Potsdam) which was a part of the German kingdom. Calling oneself king when you are citizen of a kingdom is a obvious offence of one's king. So elector Frederick III. of Brandenburg and the German king agreed on the term "King in Prussia".

After the Prussian victories in the wars of 1740/42, 1744/45 and 1756/63 against Habsburg Austria (German kings were mostly Habsburg in the 18th century) and after the annexation of West Prussia (linking Brandenburg with Hohenzollern East Prussia) from Poland in 1772, the Prussian kings carried the title "King of Prussia".