In 1854, Sir Charles Wheatstone invented the cipher known as "Playfair," named for his friend Lyon Playfair, first Baron Playfair of St. Andrews, who popularized and promoted the cipher.

Note also that, like the Enigma cipher, no letter in Playfair can be encrypted to itself—this is a direct consequence of the rules of encryption. In addition, if a digraph AB is encrypted to XY ...