ALL PERSONS HAVE BEEN ORDERED TO LEAVE FORTIFIED AREA.
In the same way the message
| 1723 | 3223 | 2825 | 1828 | 3630 | 2336 | 1423 | 2827 | 2324 | 3120 | 2317 | 3123 |
| 3036 | 2120 | 2415 | 3029 | 1512 | 2831 | 1721 | 2715 | 2811 | 2715 | 1923 | 3030 |
| 1215 | 1130 | 2128 | 3623 |
is found to be made up entirely of numbers between 11 and 36 with the numbers 23, 28 and 30 occurring most frequently. This immediately suggests an alphabet made up of the numbers from 11 to 36 inclusive and each cipher group of figures represents two letters. By arbitrary substitution of letters for groups of two numbers we obtain:
| AB | CB | DE | FD | GH | BG | IB | DJ | BK | LM | BA | LB |
| HG | NM | OP | HQ | PR | DL | AN | JP | DS | JP | TB | HH |
| RP | SH | ND | GB |
and this message is also in shape to handle as Case 6-a. It reads, on solution,
SEVEN HUNDRED MEN LEFT YESTERDAY FOR POINTS ON LOWER RIO GRANDE.
Chapter VII
We will now consider the class of substitution ciphers where a number of alphabets are used, the number and choice of alphabets depending on a key word or equivalent and being used periodically throughout the message.