Columbia subject headings:
Harvard subject headings:
Coal mines and mining--Germany--Dortmund--History.
Coal trade--Germany--Dortmund--History.
Hörde (Dortmund, Germany)--History.
Princeton subject headings:
Coal mines and mining--Germany--Dortmund--History.
Coal trade--Germany--Dortmund--History.
Hörde (Dortmund, Germany)--History.