order; after each trigram are 2 numbers, the first giving the number of the column in the Spaltenliste in which the trigram occurs, the second giving the position of the trigram in the column.
By means of a Zuteilungsliste the columns of the K book are divided amongst the various keys, the large keys being given several blocks of columns, small keys as few as 10. The K book is a large document which has probably changed only once - the current addition having come into force in 1941 - but the Zuteilungsliste was changed fairly frequently.
A set of bigram tables consisted of 9 tables, each giving a series of equivalents for the 676 existing bigrams. These tables were reciprocal, i.e., if AN = OD then OD = AN, a useful property as we shall see later. Which bigram table was in force on any given day was determined by means of a calendar which was issued with the tables. New sets of bigram tables were introduced in June 1941, November 1941, March 1943, July 1944.
Now that we are familiar with all the necessary documents - key sheets, K book, bigram tables, etc., it will be profitable to follow in detail the steps taken by a German operator wishing to send a message.
He is on board a U Boat and has Shark keys and, after consulting his Zuteilungsliste, goes to columns 272-281 which has been allotted to Shark. Here he selects the trigram H N H to serve as his Schluessel kenngruppe, He then selects from anywhere in the book another trigram (P G B) and writes them down like this:
. H N H
P G B .
He then fills in the 2 blanks with dummy letters of his own choice.
Q H N H
P G B L
and, taking the bigram table which is in force at the time, substitutes for each vertical pair of letters QP, HG, etc.,
I D Y B
N S O I
The indicator groups of his message will then be IDYB NSOI.