Columbia subject headings:
Harvard subject headings:
Ruhr (Germany : Region)--Civilization--History.
Ruhr (Germany : Region)--Economic conditions
Ruhr (Germany : Region)--Social conditions
Ruhr (Germany : Region)--Social conditions--History.
Ruhr (Germany : Region)--Social life and customs--History.
Princeton subject headings:
Ruhr (Germany : Region)--Civilization--History.
Ruhr (Germany : Region)--Social conditions--History.
Ruhr (Germany : Region)--Social life and customs--History.