Another ingenious cipher is called the “Chess Board.” First, a sheet of paper is ruled into squares exactly like a chess board--that is, a square divided into eighths each way. This arrangement gives, of course, sixty-four small squares. Then, by agreement between the people who intend to use this cipher, sixteen of these squares are agreed upon and are cut out of the sheet with a knife. Suppose, for example, this pattern is chosen: [Illustration] and the squares showing in white are cut out. Next, another sheet of paper is ruled into a chess board, of exactly the same size as the first. The perforated sheet is now laid on top of the second sheet, so that the squares on the one exactly cover the squares on the other. Now, with a pen or pencil, the plain text of the secret message is printed on the under sheet by writing through the perforations of the upper sheet, only one letter being written in each square. This, of course, permits the writing of sixteen letters of the message. Suppose the complete message is to be: “Authorize payment ten million dollars to buy copper for shipment to Germany.” Then the lower sheet, after we have written through the perforations, will look like this: [Illustration] The perforated sheet is now turned to the right through one fourth of a complete revolution, so that the top of it is at the right side of the lower sheet and so that the two chess boards again “match up.” This operation exposes, through the perforations, a new set of sixteen open squares on the lower sheet. The writing of the message is continued, and the lower sheet now looks like this (left): [Illustration] Again the perforated sheet is turned to the right, and sixteen more letters are written. Once more, and the whole sixty-four squares are utilized, looking like the last cut on the previous page. These letters are now put upright, like this. [Illustration] They are now read from left to right and from the first line down, like ordinary reading matter. They are then grouped into fives for telegraphic transmission, and an _X_ added at the end to make an even five-group there. Thus the message, as transmitted, reads: SADUL RRYAL TOHOF TRLNO IRNEI MZNPI EEIPE PGOMC APYTU LAYHM EBOOM NRNOT TESTX When this message is received, it can, of course, be quickly deciphered by printing it out on a chess board and placing over it a sheet perforated according to the prearranged pattern. This survey of codes and ciphers does not more than scratch the surface of the subject, nor more than suggest the almost infinite variations that are possible--in ciphers especially. It simply gives a groundwork for an understanding of the German secret messages now to be described. [Illustration: EXTRACTS FROM A GERMAN CODE EXPERT’S BLOTTER Showing the use of capital letters in the actual work of enciphering a message, and the combined use of cipher and code]