The trigram which he chose at random (Verfahrenkenngruppe) now provides him with the starting position of his message. To obtain this he sets up the Grundstellung in the window of his machine and taps out P G B and the three encyphered letters which result as the set-up for his message.
The first step in decyphering the message is of course to decypher the indicator groups by means of the bigram tables. At this stage the Schluesselkenngruppe can be looked up in the K-book and it can then be established by each station whether the message is on a key which it possesses. The Grundstellung is then set up, the trigram tapped out, and the message decoded.
Some form of Grundstellung procedure was common to all keys, but there were a number of keys not using bigram tables and K-book. In the Mediterranean area Kenngruppenverfahren Sued was used and discrimination between keys and dependent on the first letters of the first and second groups - the resulting bigram indicating the key. In this case the operator chose any trigram he wished and encyphered it twice at the Grundstellung and the resulting 6 letters formed the last three letters of the first 2 groups. Of this indicating system and its gradual abandonment we shall have much to say later. Of the other keys, Bonito and Bounce relied for recognition on the fact that in external appearance their traffic was unlike any other and so used no discriminating procedure, simply sending as indicators a trigram encyphered at the Grundstellung and filling the groups up with dummy letters as required.
This machine was in general use by the German Navy in all parts of the world; it was used alike for communication between ship and shore and shore and shore and by all vessels from mine sweepers and MTBs up to U Boats and major units.
The history of Hut 8 is conditioned very largely by the rapid growth of the German Naval communications system and the resulting increases of traffic and increasing number of keys. As we follow events from 1941 to 1945 I shall frequently have occasion to draw attention to this fact and it is not necessary to go into details here, but a few figures will illustrate very clearly that the gradual contraction of German occupied territory in no way signified a decrease of traffic and a simplification of the problem. Our traffic figures do not go back to 1940 but the following are the daily averages of messages for March 1941 - 1945