http://practicalcryptography.com/ciphers/porta-cipher/

The Porta cipher is a polyalphabetic substitution cipher invented by Giovanni Battista della Porta. While the Vigenere cipher is a polyalphabetic cipher with 26 alphabets, the Porta cipher is basically the same, except that it uses only 13 alphabets.

The 13 alphabets used are reciprocal, i.e. the ciphering is the same as the deciphering.