King of Prussia

Following the third Northern War, a series of treaties freed the Duchy of Prussia from vassalage to any other state, making it a fully sovereign Duchy in its own right. This complex situation (where the Hohenzollern ruler of the independent Duchy of Prussia was also a subject of the Holy Roman Emperor as Elector of Brandenburg) laid the eventual groundwork for the establishment of the Kingdom of Prussia in 1701. For diplomatic reasons, the rulers of Prussia called themselves King in Prussia from 1701 to 1772. They still nominally owed fealty to the Emperor as Electors of Brandenburg, so the "King in Prussia" title (as opposed to "King of Prussia") avoided offending the Emperor. Additionally, calling themselves "King of Prussia" implied sovereignty over the entire Prussian region, parts of which were still part of Poland.