 Because any repeated characters in the plaintext would always generate •, and similarly ∆formula_21 ⊕ ∆formula_22 would generate • whenever the "psi" wheels did not move on, and about half of the time when they did – some 70% overall.As well as applying differencing to the full 5-bit characters of the ITA2 code, Tutte applied it to the individual impulses (bits).The current "chi" wheel cam settings needed to have been established to allow the relevant sequence of characters of the "chi" wheels to be generated.