Homophonic substitution ciphers
Homophonic substitution ciphers work by replacing each plaintext character by another character, number, word or even graphic symbol. To decode ciphertext letters, one should use the reversed substitution and change characters in the other side.
The main motivation of introducing such types of ciphers was a possibility to obscure frequencies of ciphertext characters. Usually popular letters are replaced by a few other characters, numbers or phrases. Different replacements are used randomly thus frequency analysis is much more difficult.
Because of the fact that all 26 letters of the Latin alphabet should be replaced by a few corresponding phrases, the most popular technique is to assign a few numbers to each letter. One can also expand the alphabet and add a few new characters, for example by assigning different meanings to small and large letters, writing letters upside down or inventing new graphic symbols.