Columbia subject headings:
Harvard subject headings:
Bombing, Aerial--Germany--Ruhr (Region)--History.
Great Britain. Royal Air Force. Bomber Command--History.
Ruhr (Germany : Region )--History, Military--20th century.
Ruhr (Germany : Region)--History, Military--20th century.
World War, 1939-1945--Aerial operations, British.
World War, 1939-1945--Destruction and pillage--Germany--Ruhr (Region)--History.
World War, 1939-1945--Destruction and pillage--Germany.
Princeton subject headings:
Bombing, Aerial--Germany--Ruhr (Region)--History.
Great Britain. Royal Air Force. Bomber Command--History.
Ruhr (Germany : Region)--History, Military--20th century.
World War, 1939-1945--Aerial operations, British.
World War, 1939-1945--Destruction and pillage--Germany.