Columbia subject headings:
Harvard subject headings:
Immigrants--Germany--Ruhr (Region)--History--Exhibitions.
Polish people--Germany--Ruhr (Region)--History--Exhibitions.
Polish people--Germany--Ruhr (Region)--Social conditions--Exhibitions.
Ruhr (Germany : Region)--Emigration and immigration--History--Exhibitions.
Princeton subject headings:
Poland.
Polish people--Germany--Ruhr (Region).
Ruhr (Germany : Region)