That it may be possible, however, for the reader to remember this scheme of variation with less risk of discovery, it would be the better plan to distinguish the four books, not by numbers, but by letters, so that the first book should be represented by A, the second by L, the third by S, and the fourth by V.  Then it would be the case that, instead of by numbers, the variation given above would be signified by these letters, arranged in different combinations as follows.

Further, that it may not be necessary, to the end that the reader should know what order of books the writer has followed, to place these four letters at the head of each set of three letters transposed in accordance with the four books, let the writer as well as the reader not only have at hand the Stations of these four letters, indicating the order of the whole process, but also above all, see that, according to previous arrangement, there appear as a key, over each Station a combination of two letters, differing from the combination appearing over every other Station.  Thus, for example, in the scheme given above there appears in each case a group of two letters, all the groups taken together forming the words Veruntamen vani filii Hominum: Mendaces filii hominum.  If, therefore, the writer use first the third book, secondly the first book, then the second book, and finally the fourth book, he will first put at the head of his writing these two key letters, Ve: then let Transposition take place according to the designated order.  This key being learned, and the order of the books noted, the reader will easily be able to solve the secret; for, of the first book the sign is A; of the second, L; of the third, S; of the fourth, V. 

Finally, several letters of fixed number may, by agreement, be placed at the beginning of the symbolized form of the secret, or introduced in its course, at the close of some group of words, or added at the end, and this feature again contributes to the obscurity of the secret.