Columbia subject headings:
Socialism--Congresses.
Harvard subject headings:
Dialectical materialism--History--20th century--Congresses.
Socialism--Europe--20th century--Congresses.
Socialism--Europe--History--20th century--Congresses.
Princeton subject headings:
Dialectical materialism--History--20th century--Congresses.
Socialism--Europe--History--20th century--Congresses.