Editing Polybius square (section)

=== Telegraphy ===
[[File:Torches.png|alt=|thumb|Diagram of a fire signal using the Polybius cipher]]
In his ''Histories,'' Polybius outlines the need for effective signalling in warfare, leading to the development of the square. Previously, fire-signalling was useful only for expected, predetermined messages, with no way to convey novel messages about unexpected events.<ref name=":1" /> According to Polybius, in the 4th century BCE, [[Aeneas Tacticus]] devised a [[Hydraulic telegraph#Greek hydraulic semaphore system|hydraulic semaphore system]] consisting of matching vessels with sectioned rods labelled with different messages such as "Heavy Infantry", "Ships", and "Corn".<ref name=":1" /> This system was slightly better than the basic fire-signalling, but still lacked the ability to convey any needed message. The Polybius square was used to aid in telegraphy, specifically fire-signalling. To send a message, the sender would initially hold up two torches and wait for the recipient to do the same to signal that they were ready to receive the message.<ref name=":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. The sender would then raise a set of torches on his right side to indicate which letter on the tablet was intended for the message.<ref name=":1" /> Both parties would need the same tablets, a telescope (a tube to narrow view, no real magnification), and torches.<ref name=":1" />

The Polybius square has also been used in the form of the "[[knock code]]" to signal messages between cells in [[Prison|prisons]] by tapping the numbers on pipes or walls.<ref name=":0" />  It is said to have been used by [[Nihilist movement|nihilist]] prisoners of the [[Russia|Russian]] [[Czar|Czars]] and also by [[US]] [[prisoners of war]] during the [[Vietnam War]].<ref name=":32">{{cite web|url=http://crypto.interactive-maths.com/uploads/1/1/3/4/11345755/polybius_square.pdf|title=Cryptography Worksheet — Polybius Square|last=Daniel Rodriguez-Clark|website=Crypto Corner|pages=1-3<!--endif p.totales-->}}&nbsp;</ref>

[[Arthur Koestler]] describes the code being used by political prisoners of [[Stalin]] in the 1930s in his anti-totalitarian novel ''[[Darkness at Noon]]''. (Koestler had been a prisoner-of-war during the [[Spanish Civil War]].) 
Indeed, it can be signalled in many simple ways (flashing lamps, blasts of sound, [[jungle drums|drums]], [[smoke signal]]s) and is much easier to learn than more sophisticated codes like the [[Morse code]]. However, it is also somewhat less efficient than more complex codes.