As letter ciphers are the more usual, and certainly the handier of the two classes, examples are given of some systems which have been successfully used at different times and for different purposes.
The simplest of all methods, and, for that matter, the easiest to be detected, consists in having an arbitrary list of numbers, one of which shall represent each letter in the alphabet—e.g., A appears as 4, B as 8, C as 12, &c.
This plan can be varied by substituting letters for the numbers, and having each letter of the alphabet represented by another letter—e.g., A being substituted by G, B by L, C by Q, and so on; but the disadvantages attending these very simple ciphers are so great that for a message of any real importance the system is useless.
In the same way the expedient of reversing the alphabet and making A represented by Z, B by Y, C by X, is too simple and generally known to require further description.
One of the easiest and earliest ciphers is shown in [Fig. 1]. This is written in the following manner: The “bounding” lines in which the desired letters are contained are drawn and the position of the letter in them indicated by a dot. Taking, for example, [Fig. 1], A would be one dot, B two, and C three dots inscribed inside the two lines forming the angle. Thus the word CIPHER would be written
