Polybius square

[1] The device is used for fractionating plaintext characters so that they can be represented by a smaller set of symbols, which is useful for telegraphy, steganography, and cryptography.

The device was originally used for fire signalling, allowing for the coded transmission of any message, not just a finite number of predetermined options as was the convention before.

A 6 × 6 grid is also usually used for the Cyrillic alphabet (the most common variant has 33 letters, but some have up to 37) [citation needed] or Japanese hiragana (see cryptography in Japan).

In his Histories, Polybius outlines the need for effective signalling in warfare, leading to the development of the square.

[1] According to Polybius, in the 4th century BCE, Aeneas Tacticus devised a hydraulic semaphore system consisting of matching vessels with sectioned rods labelled with different messages such as "Heavy Infantry", "Ships", and "Corn".

[1] The sender would then hold up the first set of torches on his left side to indicate to the recipient which tablet (or row of the square) was to be consulted.

[1] Both parties would need the same tablets, a telescope (a tube to narrow view, no real magnification), and torches.

[1] The Polybius square has also been used in the form of the "knock code" to signal messages between cells in prisons by tapping the numbers on pipes or walls.

[3] Arthur Koestler describes the code being used by political prisoners of Stalin in the 1930s in his anti-totalitarian novel Darkness at Noon.