Banbury Sheet

Among the fragmented and torn wartime documents relating to Codebreaking processes that were discovered in the roof space of Hut 6 there were the remains of 2 Banbury Sheets.

'Banburismus' was a method of Enigma decryption devised by Alan Turing in 1940 to overcome the lack of cribs in Naval Enigma. The process involved looking for messages with elements in common that might help to focus menus and reduce Bombe time.

Banbury Sheets were long sheets of strong paper that were manufactured in the Oxfordshire town of Banbury. These sheets had columns of letters printed vertically and clerks punched out holes to correspond with letters in the Enigma message. When 2 Banbury sheets were superimposed on one another, identical letters in the same position were readily seen by the light which came through coincident holes, providing an indication of possible “depth”.

When in February 1942 the German Navy introduced a modified Enigma machine that was fitted with a fourth rotor, codenamed “Shark”, this codebreaking method became useless.