Columbia subject headings:
Referendum--Congresses.
Referendum--Europe--Congresses.
Referendum--United States--Congresses.
Harvard subject headings:
Referendum--Congresses.
Referendum--Europe--Congresses.
Referendum--United States--Congresses.
Princeton subject headings:
Referendum--Congresses.
Referendum--Europe--Congresses.
Referendum--United States--Congresses.