Columbia subject headings:
Harvard subject headings:
Iron industry and trade--Germany--Ruhr River Valley.
Iron industry and trade--Germany--Westphalia.
Steel industry and trade--Germany--Ruhr River Valley.
Steel industry and trade--Germany--Westphalia.
Princeton subject headings:
Iron industry and trade--Germany--Ruhr River Valley.
Iron industry and trade--Germany--Westphalia.
Steel industry and trade--Germany--Ruhr River Valley.
Steel industry and trade--Germany--Westphalia.