Columbia subject headings:
Harvard subject headings:
Coal miners--Labor unions--Germany--Ruhr (Region)--History.
Miners--Labor unions--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Politics and government--20th century.
Princeton subject headings:
Coal miners--Labor unions--Germany--Ruhr (Region)--History.
Miners--Labor unions--Germany--Ruhr (Region)--History.
Ruhr (Germany : Region)--Politics and government--20th century.