Polybius square

The Polybius square, also called the Polybius checkerboard, is a device for fractionating plaintext characters so that each one can be represented by a smaller set of symbols. That property makes it useful in telegraphy, steganography and cryptography. It was invented by the ancient Greeks Cleoxenus and Democleitus and made famous by the historian and scholar Polybius, whose name it carries. The device was originally used for fire signalling, where it allowed any message at all to be transmitted in code, not just one of a finite number of predetermined messages, which had been the convention before. Its central idea, replacing each letter with a short coordinate pair drawn from a small alphabet of symbols, recurs in later ciphers that build on fractionation.