Columbia subject headings:
Harvard subject headings:
Economic development--Germany--Ruhr (Region)--History.
Industries--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Economic conditions
Ruhr (Germany : Region)--History
Princeton subject headings:
Economic development--Germany--Ruhr (Region)--History.
Industries--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Economic conditions.
Ruhr (Germany : Region)--History.