General Report on Tunny

Page 3

-------------------------------
11 - GERMAN TUNNY.
-------------------------------

11A FISH MACHINES.

(a) The Teleprinter Alphabet.

Two teleprinters in communication consist of two enlarged electromatic typewriters connected by cable, and constructed so that whatever is typed on either keyboard is printed on both typewriters. When a key is depressed by the sender, the enlarged typewriter sends along the cable one of 32 electrical signals. These signals consist of five consecutive impulses, each of which may be positive (known as DOT) or negative (known as CROSS) and they operate the appropriate key of the receiving typewriter. The 32 signals are known as 'LETTERS' and correspond to the keys on the teleprinter keyboard.

It is clear that the number of keys cannot be greater than 32, and it is in fact 31. However 29 out of 31 keys can have two meanings, one in figure shift and one in letter shift, the remaining two being used to operate the change to letter shift and the change to figure shift respectively.

The following table shows the construction and meanings of the letters in the teleprint alphabet - as laid down by international convention. Figure shift meanings are liable to variation when they have a purely national significance (e.g. £). The order of the letters is specially devised for cryptographic purposes and not conventional.