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The comparative strength of a ΔΨ' and a ΔD count can be seen in Fig. 12 (II) and is such that whereas chis are rarely broken from under 4000 letters of Z = Χ + D, they can sometimes be broken from 100 letters of K = Χ + Ψ'. This means that the dimensions of the job make it quite practicable by hand though a Colossus may profitably be used if the stretch of key is sufficiently long.
Wheelbreaking from key on Χ2 limitation normally starts with a count or run. Wheel-breaking from other kinds of key normally starts with KEY RECTANGLES and a COMBINED (ΔΧ5) FLAG. If this is significant the chi patterns obtained are used to complete the wheels by an improved form of TURINGERY (Turing's Method), or alternatively by Colossus wheel-breaking methods, if the key has more than 300 letters.
Once the chi patterns have been found and the ΔΨ' stream obtained, the recovery of the psi patterns is trivial.