Columbia subject headings:
World War, 1914-1918--Cryptography.
World War, 1914-1918--Naval operations.
World War, 1914-1918--Secret service--Great Britain.
Harvard subject headings:
World War, 1914-1918--Cryptography
World War, 1914-1918--Naval operations
World War, 1914-1918--Secret service--Great Britain
Princeton subject headings:
World War, 1914-1918--Cryptography.
World War, 1914-1918--Naval operations.
World War, 1914-1918--Secret service--Great Britain.