Columbia subject headings:
Mineral industries--Government policy--Germany--Ruhr (Region)--History.
Miners--Labor unions--Germany--Ruhr (Region)--History.
Harvard subject headings:
Mineral industries--Government policy--Germany--Ruhr (Region)--History.
Miners--Labor unions--Germany--Ruhr (Region)--History.
Princeton subject headings:
Coal miners--Germany (West)--Ruhr (Region)--History.
Coal mines and mining--Germany (West)--Ruhr (Region)--History.