bigrams and trigrams. Tetragrams often represented German words in the plaintext and their scores were calculated according to the type of message (from traffic analysis), and even their position within the message. These were tabulated and the relevant values summed by Banburists in assessing pairs of messages to see which were likely to be in depth. Bletchley Park used the convention that the indicator plaintext of "VFX", being eight characters ahead of "VFG", or (in terms of just the third, differing, letter) that "X = G+8". Scritchmus Scritchmus was the part of the Banburismus procedure that could lead to the identification of the right-hand (fast) wheel. The Banburist might have evidence from various message-pairs (with only the third indicator letter differing) showing that "X = Q−2", "H = X−4" and "B = G+3". He would search the deciban sheets for all distances with odds of better than 1:1 (i.e. with scores ≥ +34). An attempt was then made to construct the 'end wheel alphabet' by forming 'chains' of end-wheel letters out of these repeats. They could then construct a "chain" as follows: G--B-H---X-Q If this is then compared at progressive offsets with the known letter-sequence of an Enigma rotor, quite a few possibilities are discounted due to violating either the "reciprocal" property or the "no-self-ciphering" property of the Enigma machine: G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (G enciphers to B, yet B enciphers to E) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (H apparently enciphers to H) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (G enciphers to D, yet B enciphers to G) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (B enciphers to H, yet H enciphers to J) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (Q apparently enciphers to Q) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (G apparently enciphers to G) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (G enciphers to H, yet H enciphers to M) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (H enciphers to Q, yet Q enciphers to W) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (X enciphers to V, yet Q enciphers to X) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (B enciphers to Q, yet Q enciphers to Y) G--B-H---X-Q ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (X enciphers to X) Q G--B-H---X-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible -Q G--B-H---X-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (Q enciphers to B, yet B enciphers to T) X-Q G--B-H--->