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A few definitions of special terms will help to make that text clearer.

A cipher is a method of writing a message so that it cannot be read by anyone ignorant of the method.

A cryptogram is a message written in cipher.

The clear is the communication which it is desired to make.

The message is that communication after it has been written in cipher.

A substitution cipher is one in which letters of the clear are replaced by letters, figures or symbols.

A simple-substitution cipher is one in which one letter of the clear is represented by one, and always the same, letter, figure or symbol of the cipher. Example: if the clear be “Come here” and each letter be represented by the one following it in the alphabet, the resulting message will read DPNF IFSF.

A simple-substitution cipher with suppression of frequencies is one in which each of the very common letters (such as E) is represented by several figures or symbols. Example: the same clear as above, “Come here,” with each letter still represented by the one following it in the alphabet. But it has been concerted that in addition to F, the figures 2 and 3 will also represent E. The message now reads DPNF I2S3.

A double-substitution cipher is one in which letters of the clear are represented in the cipher by letters which vary according to a system, the basis of which is a key-word. To be more fully explained and illustrated in the text. See Chapter Six.

A two-step cipher, not at all the same thing as the last, is one in which the message, usually obtained by enciphering by simple or double substitution, is now enciphered for a second time. The second substitution is usually made according to a table, of which both sender and receiver have copies. This table may (for instance) make the second substitution on the basis of two or more letters at a time. Example: the same “Come here,” enciphered as DPNF IFSF. The cipherer refers to his table; suppose it indicates as the value for DP, 416; 317 as that for NF; 96 for IF and 138 for SF. The message would then be 416 317 96 138. In two-step ciphers one of the steps is usually substitution, the other transposition.

A transposition cipher is one in which the letters remain the same as in the clear, but are shuffled according to a prearranged pattern. Example: the same clear as above, written in two lines:

which is read off line by line, the message sent being CMHR OEEE.

A combination cipher is a two-step cipher in which the steps are transposition and substitution. Example: “Come here,” enciphered by substitution to DPNF IFSF, which is now written:

and the message taken off as DNIS PFFF.

A grill cipher is one in which a grill or mask with holes in it is placed over the paper on which the message is to be written. The message, in clear, is then written through the holes; the grill removed and the spaces between the words or letters filled up with others to give the whole the appearance of an innocent letter containing no cipher message.

A syllable cipher is one in which substitution or transposition is made on the basis of syllable or pairs or triplets of letters instead of single letters. This is very rare.

Nulls are letters or words having no connection with the clear, introduced to confuse a decipherer.

Stops are punctuation marks, usually sentence endings, for which special characters are provided, sometimes placed after each word.

To encipher a message is to put it in cipher. To decipher it is to reduce it to clear. To break a cipher or code is to discover the system by which it was composed. It is perfectly possible for a cryptographer to read the content of a ciphered message without being able to discover the system on which it was written. Another message in the same cipher forces him to repeat the work of decipherment.

Frequency tables are the most important tools of the cryptographer. They are tables showing the relative frequencies of letters, pairs of letters, triplets (trigrams), syllables or words in normal text.