Columbia subject headings:
Communism--Europe--History--20th century--Congresses.
Socialism--Europe--History--20th century--Congresses.
Socialist parties--Europe--History--20th century--Congresses.
Harvard subject headings:
Communism--Europe--History--20th century--Congresses.
Socialism--Europe--20th century--Congresses.
Socialism--Europe--History--20th century--Congresses.
Socialist parties--Europe--History--20th century--Congresses.
Princeton subject headings:
Communism--Europe--History--20th century--Congresses.
Socialism--Europe--History--20th century--Congresses.
Socialist parties--Europe--History--20th century--Congresses.