Columbia subject headings:
Harvard subject headings:
Catholic Church--Foreign relations--Holy Roman Empire.
Frederick II, Holy Roman Emperor, 1194-1250.
Gregory IX, Pope, approximately 1170-1241.
Holy Roman Empire--Foreign relations--Catholic Church.
Princeton subject headings:
Church and state.
Germany--History--Frederick II, 1215-1250.
Holy Roman Empire--History--Frederick II, 1215-1250.