The method of perforated sheets was a codebreaking technique used against the Enigma machine (see Cryptanalysis of the Enigma). The method involved superimposing a series of sheets — each containing a grid of holes in various positions — and shining a lamp underneath; a large number of possibilities for the secret Enigma settings could be eliminated using this approach. It was first developed in 1938 in Poland by Henryk Zygalski, and accordingly is sometimes known as Zygalski sheets. They were also put to use in Bletchley Park in England, where they were manufactured by John Jeffreys.
In May 1940, the Germans changed their Enigma indicating procedure (with the exception of a Norwegian key), obsoleting the attack based on perforated sheets.
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