This code simply maps each letter of the alphabet in the message to be encoded onto the reverse letter of the alphabet, so 'a' becomes 'z', 'b' becomes 'y', and so on up to 'z' becomes 'a'. Non alphabetic characters are left unchanged.

This code increments each letter of the alphabet by the same number of characters, so if the number to rotate was '3' for example, 'a' becomes 'd', 'b' becomes 'e', etc. Non alphabetic characters are left unchanged. A more sophisticated incrementing of letters occurs in the Variable Alphabet Mapping Code.

This code simply maps each letter of the alphabet onto another letter using a keyword. to work out what letter is used for each mapping the keyword is first written (removing any duplicate letters) followed by the remaining letters of the alphabet in order, giving a 26 letter sequence which is used to map the 26 letters of the alphabet. Non alphabetic characters are left unchanged.

For example, if the keyword was 'herbert', the letters 'herbt' would be written (ignoring the duplicate 'e' and 'r') followed by the remaining letters in order, i.e. 'acdfgijklmnopqsuvwxyz', giving 'herbtacdfgijklmnopqsuvwxyz'. We now have a one to one mapping from abcdefghijklmnopqrstuvwxyz to herbtacdfgijklmnopqsuvwxyz, so 'a' becomes 'h', 'b' becomes 'e', ... 'f' becomes 'a', 'g' becomes 'c', etc.

This code uses a numeric string as the key, with each letter in the message to be encoded being incremented by the number of letters indicated by a digit in the numeric key. The first letter is incremented using the first digit, the second letter is incremented using the second digit, and so on. Once the last digit of the key has been reached, we return to the first digit of the key. Non alphabetic characters are left unchanged. The numeric key could typically be something easy to remember such as a date (e.g. '14031957').

This code is a geometrical substitution code, where each letter of the alphabet is represented by a symbol which is part of a grid or a cross in which the letters are contained.