 The resultant stream (symbolised by the Greek letter "delta" Δ) was called the difference because XOR is the same as modulo 2 subtraction.The reason that this provided a way into Tunny was that although the frequency distribution of characters in the ciphertext could not be distinguished from a random stream, the same was not true for a version of the ciphertext from which the "chi" element of the key had been removed.This was the case because where the plaintext contained a repeated character and the "psi" wheels did not move on, the differenced "psi" character (Δformula_2) would be the null character ('/ ' at Bletchley Park).