Columbia subject headings:
Church and state--History.
Frederick II, Holy Roman Emperor, 1194-1250.
Gregory IX, Pope, approximately 1170-1241.
Harvard subject headings:
Church and state--History
Federick II, Holy Roman Emperor, 1194-1250.
Gregory IX, Pope, approximately 1170-1241.
Princeton subject headings:
Church and state--History.
Frederick II, Holy Roman Emperor, 1194-1250.
Gregory IX, Pope, approximately 1170-1241.