Second requisite: besides a code, as above, ciphers are used, illegal parts of letters being put not only into code but also into cipher. There are many different systems of cipher. The simplest and at the same time most reliable system of cipher is the system of cipher by the help of a book. Some book or other is agreed upon beforehand and then the cipher is made in this way: simple fractions or decimals are ciphered. The first figure of the first fraction shows the page of the book. Then further comes the actual cipher. For the numerator of the fraction we must take a line counting from above or below; for the denominator that counting from the left or from the right which it is necessary to put into cipher. For example, we need to put into cipher the letter “A”. We look in the book and we see that this letter is in the third line from the top, the fourth letter from the left to the right. Then we cipher 3 over 4 (3/4), that is the third line from the top, fourth letter from left to right. You can agree also on this method; for example, counting the line not from above but from below, then the 3 will not be the third line from above but the third line from below. You can agree to count the letter in the line not from left to right but from right to left. Finally, for greater complexity in order to keep the sense from the police, you can also add to the fraction some figure or other. Let us say the numerator is increased by 3 and the denominator by 4. In this case in order to decipher, it will be necessary first to subtract in the numerator and denominator of every fraction. A whole number of similar complications can be thought out in order to complicate the cipher. The advantage of such a cipher is that it is not only very simple but also that each letter can be designated by a great number of different signs and in such a way that the cipher designation of the letters are not repeated. The book cipher can be used without a book. In place of a book some poem or other can be chosen, learned by heart and the ciphering done according to it. When it is necessary to cipher or decipher, the poem must be written out in verses and then the ciphering or deciphering done and the poem destroyed.