Columbia subject headings:
World War, 1939-1945--Campaigns--North Atlantic Ocean.
World War, 1939-1945--Naval operations, German.
World War, 1939-1945--Naval operations--Submarine.
Harvard subject headings:
World War, 1939-1945--Campaigns--North Atlantic Ocean.
World War, 1939-1945--Naval operations, German.
World War, 1939-1945--Naval operations--Submarine.
Princeton subject headings:
World War, 1939-1945--Campaigns--North Atlantic Ocean.
World War, 1939-1945--Naval operations, German.
World War, 1939-1945--Naval operations--Submarine.