King in Prussia
King in Prussia was a title used by the Electors of Brandenburg from 1701 to 1772. Subsequently they used the title King of Prussia.
The House of Hohenzollern ruled Brandenburg as Prince-Electors, and were subjects of the Holy Roman Emperor. Since 1618, the Electors of Brandenburg had also ruled the Duchy of Prussia, which lay outside of the empire, in a personal union. The dual state was known as Brandenburg-Prussia. The Duchy of Prussia was originally a fief that the dukes held of the King of Poland, of whom they were vassals, until the Treaties of Labiau (1656) and Bromberg (1657), with which Frederick William I, the Great Elector, had achieved full sovereignty from the Polish Crown. In 1701 Elector Frederick III wanted to show his greatness by adopting the title king. At the time, only three royal titles were permitted within the empire: "King of the Germans" (a title held by the emperor), "King of Bohemia" (often held by the emperor as well), and "King of the Romans" (held by the emperor's heir).
In return for Hohenzollern assistance in the War of the Spanish Succession and support for the Habsburg candidate in the subsequent election, Emperor Leopold I allowed Frederick to crown himself "King in Prussia", not "King of Prussia"; Leopold accepted Frederick's argument that since Prussia had never been part of the empire and the Hohenzollerns were fully sovereign over it, it could be ruled as a kingdom.
Legally, Frederick was only an elector in his domains within the borders of the empire; he was only king in his former duchy. However, the emperor's power had become only nominal by this time, and his ultimate overlordship over Brandenburg and the rest of the empire had largely become a legal fiction. Hence, even though Brandenburg was still legally part of the empire and ruled in personal union with Prussia, it soon came to be treated as a de facto part of Prussia.
On 17 January 1701, Frederick dedicated the royal coat of arms, the Prussian black eagle with the motto "suum cuique" imprinted. On 18 January, he crowned himself and his wife Sophie Charlotte in a baroque ceremony in Königsberg Castle.
Even so, Frederick's move was controversial, and only became widely accepted after the Treaty of Utrecht in 1713. The title "King of Prussia" implied lordship over the entire Prussian region, not simply the former Duchy of Prussia, now Kingdom of Prussia, and the assumption of such a title by the Hohenzollern margraves would have threatened neighboring Poland; because the province of Royal Prussia was part of the Kingdom of Poland, the Kings of Poland titled themselves Kings of Prussia until 1742.
Throughout the 18th century the power of the Kings in Prussia continued to increase. They were victorious over the Austrian Habsburg Monarchy in the three Silesian Wars, greatly increasing their power through the acquisition of Silesia. King Frederick II adopted the title King of Prussia in 1772, the same year he annexed most of Royal Prussia in the First Partition of Poland. Reflecting the technicality that Brandenburg was still part of the Holy Roman Empire, the kings of Prussia continued to use the title of Elector of Brandenburg until the empire's dissolution in 1806.