Columbia subject headings:
Harvard subject headings:
Coal miners--Germany--Ruhr (Region).
Labor unions--Germany--Ruhr (Region)
Strikes and lockouts--Coal mining--Germany--Ruhr (Region)
Princeton subject headings:
Coal miners--Germany--Ruhr (Region).
Labor unions--Germany--Ruhr (Region).
Strikes and lockouts--Coal mining--Germany--Ruhr (Region).