Frederick I (German: Friedrich I.) (11 July 1657 – 25 February 1713), of the Hohenzollern dynasty, was (as Frederick III) Elector of Brandenburg (1688–1713) and Duke of Prussia in personal union (Brandenburg-Prussia). The latter function he upgraded to royalty, becoming the first King in Prussia (1701–1713). From 1707 he was in personal union the sovereign prince of the Principality of Neuchâtel (German: Fürstentum Neuenburg). He was also the paternal grandfather of Frederick the Great. Born in Königsberg, he was the third son of Frederick William, Elector of Brandenburg by his father's first marriage to Louise Henriette of Orange-Nassau, eldest daughter of Frederick Henry, Prince of Orange and Amalia of Solms-Braunfels. His maternal cousin was King William III of England. Upon the death of his father on 29 April 1688, Frederick became Elector Frederick III of Brandenburg and Duke of Prussia. The Hohenzollern state was then known as Brandenburg-Prussia. The family's main possessions were the Margraviate of Brandenburg within the Holy Roman Empire and the Duchy of Prussia outside of the Empire, ruled as a personal union. Although he was the Margrave and Prince-elector of Brandenburg and the Duke of Prussia, Frederick desired the more prestigious title of king. However, according to Germanic law at that time, no kingdoms could exist within the Holy Roman Empire, with the exception of the Kingdom of Bohemia. Frederick persuaded Leopold I, Archduke of Austria and Holy Roman Emperor, to allow Prussia to be elevated to a kingdom. This agreement was ostensibly given in exchange for an alliance against King Louis XIV in the War of the Spanish Succession. Frederick argued that Prussia had never been part of the Holy Roman Empire, and he ruled over it with full sovereignty. Frederick crowned himself on 18 January 1701 in Königsberg. Although he did so with the Emperor's consent, and also with formal acknowledgement from Augustus II the Strong, Elector of Saxony, who held the title of King of Poland, the Polish-Lithuanian Diet (Sejm) raised objections, and viewed the coronation as illegal. In fact, according to the terms of the Treaty of Wehlau and Bromberg, the House of Hohenzollern's sovereignty over the Duchy of Prussia was not absolute but contingent on the continuation of the male line (in the absence of which the duchy would revert to the Polish crown). Therefore, out of deference to the region's historic ties to the Polish crown, Frederick made the symbolic concession of calling himself "King in Prussia" instead of "King of Prussia".

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