Why was Frederick I only called "King of Denmark" while Christian III was named "King of Denmark and Norway"?
Frederick I of Denmark Frederick I (7 October 1471 – 10 April 1533) was the King of Denmark and Norway. His name is also spelled "Friedrich" in German, "Frederik" in Danish and Norwegian and "Fredrik" in Swedish. He was the penultimate Roman Catholic monarch to reign over Denmark, when subsequent monarchs embraced Lutheranism after the Protestant Reformation. As King of Norway, Frederick is most remarkable in never having visited the country and was never being crowned King of Norway. Therefore he was styled "King of Denmark, the Vends and the Goths, elected King of 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.