messages sent down were called the Freeborns and the material that came back was the Freebornery! It was sent down in batches of about 10 messages at a time and Freeborn punched up the traffic preparatory to producing the tetra catalogue. When the bulk of the traffic had reached him he started producing the catalogue and sending up results in the form shown in Para 11 (a supplement was sent up at the end to cover odd messages that had come in late). The handling of the material was reduced to such a fine art ultimately that although the last of the day's traffic did not arrive until 14.00 or 15.00 (the key running from 12.00 - 12.00) results were coming up by 17.00. [Later on, if the day was proving difficult, we had a trigram catalogue listing 3 letter repeats between pairs of messages with the first letters of the trigrams in common. This is a refinement it is unnecessary to describe more fully]. This tetra catalogue providing as it did our "good fits" was the foundation of our work and the speedy and efficient service we received played a major part in our success.
21. The Big Room did all the rest of the preparatory work. The message trigrams having been worked out (this was done by the R.R. and re-done as a check by the Big Room since it was vital to have no error) the messages were prepared for comparison as follows. We have a stock of long sheets, known as banburies since they were originally made at Banbury, having 100 - 250 columns (they were made in varying lengths) each column containing the letters A to Z in alphabetical order. If the message was say 182 letters long and the text began JXBNR ... , a girl would take a 200 long banbury and would mark in red the letters J, X, B, N, R ... in successive columns. Having done this she would take the banbury to a hand punch and punch out the marked letters. Here is a miniature punched sheet - with a 10 letter alphabet and 12 columns instead of a 26 letter alphabet and 200 columns. The "message" is CJAEBBIHGADJ and Ø represents a punched hole. Note also (1) message trigram (2) message number (3) dummyismus (i.e. estimated % chance of message being dummy) (4) category (explained later) (5) guide hole.