The first stage of encipherment is based on a key word, which must have six letters and which is changed at intervals by the Centre. In the example to follow the key word is PLAYER. This is written out horizontally, and below it- in line- the remainder of the alphabet is written out (in the case of PLAYER in groups of six) together with a symbol for "signal" (@) and Full Stop. "Signal" means that the message changes over from letters to numbers.
At this stage the "pattern" looks like this:
It is now necessary to assign a number to each letter, and for this purpose the numbers 1-9, 00-09, and 40-49 are used. (In the first set of numbers the figure 4 is omitted as it would lead to confusion with the double numbers beginning with 40.)
Those letters which are changed into single figures (1-9) are ASINTOER (arranged in this order for mnemonic purposes), the numbers being assigned to the relevant letters as they occur reading vertically and starting in the top left-hand corner of the diagram. With these numbers in place, the diagram now looks as follows:
To this pattern are now added the double numbers (00-09, 40-49), starting from the top left corner and going down the columns consecutively filling in the blanks.
With all the numbers inserted, the diagram now looks like this:
(This pattern is, of course, changed occasionally by the Centre by allocating a different "mnemonic" or by changing the code word, or both.)