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:
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):
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.
