Columbia subject headings:
Harvard subject headings:
Engels, Friedrich, 1820-1895--Congresses.
Marx, Karl, 1818-1883--Congresses.
Socialism--History--19th century--Congresses.
Princeton subject headings:
Engels, Friedrich, 1820-1895--Congresses.
Marx, Karl, 1818-1883--Congresses.
Socialism--History--19th century--Congresses.