Columbia subject headings:
Holy Roman Empire--History--Frederick I, 1152-1190.
Holy Roman Empire--History--Frederick II, 1215-1250.
Holy Roman Empire--History--Interregnum, 1254-1273.
Lombard League.
Rhine Cities, League of, 1254.
Harvard subject headings:
Holy Roman Empire--History--Frederick I, 1152-1190.
Holy Roman Empire--History--Frederick II, 1215-1250.
Holy Roman Empire--History--Interregnum, 1254-1273.
Lombard League.
Rhine Cities, League of, 1254.
Princeton subject headings:
Cities and towns, Medieval--History.
Europe--History--476-1492.