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Polybius square. Diagram illustrating the principles of the Polybius square, a system used in encryption and telegraphy, first described by the ancient Greek scholar Polybius in around 200BC. It works by replacing each letter with a pair of grid co-ordinates. For example, S would become 43. Polybius conceived of this device as a way of transmitting messages optically with a pair of torches. For an artwork of the system in practice, see image V400/0090.
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