Chapter VI
On Mutual Transposition of the Letters
of a Single Alphabet.

The second Principal Mode of Transposition takes place by Mutual Interchange of the letters of a single alphabet; (see, above, c.2 of this Book).  This Transposition is a Transposition of all the letters, one with another, and again is accomplished by three principal Modes, as follows:

The first Mode takes place when we place the letters in two lines and then interchange the letters of one line with those of the other.  E.g.

a b c d e f g h I l m
n o p q r s t u x y z

This Mode, as is shown by the next Mode, may likewise be varied, according as we arrange the letters in any one of a variety of ways.

The second Mode takes place when the letters are arranged in two alphabetic series, in the manner given by de Vigenere, p.223, and presented herewith.

Note in this case a twofold process: either, namely, the upper series is transposed, or , conversely, the lower.  For example, let the secret be Omnium rerum vicissitudo.  When the upper series is transposed, we write the secret thus:  Labtda, unuda, dtftytodgl.  When the lower series, other letters appear, as follows: Tpesrp, alarp, rsbsffsirut.  To give greater concealment to the secret, this Mode may be varied thus:  The first line, word, or letter may follow the upper series and process; the second, the lower; the third, the upper again; the fourth, the lower; and so on.

The third Mode is still more laborious, for it requires a table and a key.  Let the table be so constructed that the alphabet, divided, for the purposes of Mutual Transposition, into two equal parts, is set down eleven times, with capital letters, serving as a key, written at the sides of each order, thus:

The manner of using the table and the method of applying the key should also come up for demonstration here, but, as these points are fully treated by special method in Bk. 9.C.5.m.7, below.