Why was Frederick I only called "King of Denmark" while Christian III was named "King of Denmark and Norway"?
Election of Christian III The election of Christian III as king of Denmark and Norway on 4 July 1534 was a landmark event for all of Denmark and Norway. It took place in the church in the town of Rye, eastern Jutland, where the Jutlandic nobility elected Prince Christian, son of King Frederick I and Duke of Schleswig and Holsten, as king. This brought about the Count's Feud ("Grevens Fejde") and later also led to the implementation of the Protestant Reformation in Denmark and Norway.
Flag of Prussia The state of Prussia had its origins in the separate lands of the Margraviate of Brandenburg and of the Duchy of Prussia. The Margraviate of Brandenburg developed from the medieval Northern March of the Holy Roman Empire, passing to the House of Hohenzollern in 1415. The Duchy of Prussia originated in 1525 when Albert of Brandenburg-Ansbach, a member of a cadet branch of the Hohenzollerns, secularized the eastern lands of the Teutonic Knights as a Polish fief. Prince-elector John Sigismund, Elector of Brandenburg, inherited the Duchy of Prussia in 1618, thus uniting Brandenburg and Prussia under one ruler in a personal union; the Elector's state became known as Brandenburg-Prussia. The Kingdom of Prussia formed when Elector Frederick III assumed the title of Frederick I, King in Prussia, on 18 January 1701.