ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible -X-Q G--B-H--> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (X enciphers to B, yet B enciphers to V) --X-Q G--B-H-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible ---X-Q G--B-H-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (X enciphers to D, yet B enciphers to X) H---X-Q G--B-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (Q enciphers to G, yet G enciphers to V) -H---X-Q G--B-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (H enciphers to B, yet Q enciphers to H) B-H---X-Q G--> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible (note the G enciphers to X, X enciphers to G property) -B-H---X-Q G-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is impossible (B enciphers to B) --B-H---X-Q G-> ABCDEFGHIJKLMNOPQRSTUVWXYZ ......... is possible The so-called "end-wheel alphabet" is already limited to just nine possibilities, merely by establishing a letter-chain of five letters derived from a mere four message-pairs. Hut 8 would now try fitting other letter-chains — ones with no letters in common with the first chain — into these nine candidate end-wheel alphabets. Eventually they will hope to be left with just one candidate, maybe looking like this: NUP F----A--D---O --X-Q G--B-H-> ABCDEFGHIJKLMNOPQRSTUVWXYZ Not only this, but such an end-wheel alphabet forces the conclusion that the end wheel is in fact "Rotor I". This is because "Rotor II" would have caused a mid-wheel turnover as it stepped from "E" to "F", yet that's in the middle of the span of the letter-chain "F----A--D---O". Likewise, all the other possible mid-wheel turnovers are precluded. Rotor I does its turnover between "Q" and "R", and that's the only part of the alphabet not spanned by a chain. That the different Enigma wheels had different turnover points was, presumably, a measure by the designers of the machine to improve its security. However, this very complication allowed Bletchley Park to deduce the identity of the end wheel. Middle wheel Once the end wheel is identified, these same principles can be extended to handle the middle rotor, though with the added complexity that the search is for overlaps in message-pairs sharing just the first indicator letter, and that the overlaps could therefore occur at up to 650 characters apart. The workload of doing this is beyond manual labour, so BP punched the messages onto 80-column cards and used Hollerith machines to scan for tetragram repeats or better. That told them which banburies to set up on the light boxes (and with what overlap) to evaluate the whole repeat pattern. Armed with a set of probable