Columbia subject headings:
Harvard subject headings:
Cities and towns--Germany--Ruhr (Region)--History--20th century.
City planning--Germany--Ruhr (Region)--History--20th century.
Industrial buildings--Germany--Ruhr (Region)--History--20th century.
Princeton subject headings:
Cities and towns--Germany--Ruhr (Region)--History--20th century.
City planning--Germany--Ruhr (Region)--History--20th century.
Industrial buildings--Germany--Ruhr (Region)--History--20th century.