Columbia subject headings:
Harvard subject headings:
Coal mines and mining--Germany--Ruhr (Region)--History.
Coal trade--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--History
Princeton subject headings:
Coal mines and mining--Germany--Ruhr (Region)--History.
Coal trade--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--History.