Columbia subject headings:
Coal miners--Germany--Ruhr (Region)--History.
Coal trade--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Economic conditions.
Ruhr (Germany : Region)--Economic policy.
Harvard subject headings:
Coal miners--Germany--Ruhr (Region)--History.
Coal trade--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Economic conditions
Ruhr (Germany : Region)--Economic policy.
Princeton subject headings:
Coal miners--Germany (West)--Ruhr (Region)--History.
Coal trade--Germany (West)--Ruhr (Region)--History.
Ruhr (Germany : Region)--Economic conditions.
Ruhr (Germany : Region)--Economic policy.