Columbia subject headings:
City-states--Italy--Po River Plain--History--Congresses.
Feudalism--Italy--Po River Plain--History--Congresses.
Po River Plain (Italy)--Politics and government--Congresses.
Harvard subject headings:
City-states--Italy--Po River Plain--History--Congresses.
Feudalism--Italy--Po River Plain--History--Congresses.
Po River Plain (Italy)--Politics and government--Congresses.
Princeton subject headings:
City-states--Italy--Po River Plain--History--Congresses.
Italy--Politics and government--1268-1559--Congresses.
Italy--Politics and government--1559-1789--Congresses.