Columbia subject headings:
Harvard subject headings:
Coal miners--Germany (West)--History.
Coal miners--Great Britain--History.
Coal trade--Germany--Ruhr (Region)--History.
Coal trade--Great Britain--History.
Princeton subject headings:
Coal miners--Germany (West)--History--Congresses.
Coal miners--Great Britain--History--Congresses.
Coal trade--Germany (West)--Ruhr (Region)--History--Congresses.
Coal trade--Great Britain--History--Congresses.