If we now list the number of occurrences, throughout the day's traffic, of each of the encyphered letters in our two boxes, we can decypher them successively on each of the 5 possible alphabets and score the letters which result. If our table of occurrences in the N.Africa area looks like this:
the alphabets score respectively as follows:
In this case there can be no doubt that 2 is the right alphabet and scoring for the other areas would doubtless confirm the fact. If, as often happened, there were 2 or more candidates for the right alphabet, knowledge of trigrams normally clinched the question.
As we have already seen, trigrams were chosen freely by the operator and hence various preferences became apparent. The only generally accepted and peculiarly absurd rule seems to have been that a trigram might not ‘crash' i.e. contain any repeated letter like NZN or PPL and so any alphabet which produced a crash was probably wrong; when traffic was heavy, this method of selecting alphabets was very powerful. As well as rejecting on bad trigrams, selecting on good trigrams was also powerful; straights along the key board (like WER, the most popular trigram)