A polyphonic substitution cipher is one in which several different plaintext letters are enciphered into a single cipher letter or symbol. Perhaps the most simple and well-known example of a polyphonic substitution cipher is the telephone dial, in which the letters ABC are encoded by the number 2, DEF by 3, GHI by 4, JKL by 5, MNO by 6, PRS by 7, TUV by 8, and WXY by 9. This is quite different from the well-known (monophonic) substitution cipher, in which each plaintext letter is associated with a different cipher letter -- if A is encoded by T, then no other letter of the alphabet is also encoded by T.
Eckler, A. Ross
"A Readable Polyphonic Cipher,"
Word Ways: Vol. 8
, Article 16.
Available at: http://digitalcommons.butler.edu/wordways/vol8/iss1/16