Complex sentence: On 11 June, 1441, Frederick II married Katharina of Saxony, a daughter of Frederick I, Elector of Saxony and Katharina Braunschweig-Lüneburg.
Split into simpler sentences: On 11 June, 1441, Frederick II married Katharina of Saxony. She was a daughter of Frederick I, Elector of Saxony and Katharina Braunschweig-Lüneburg.