““Polybius, devised a system of signaling that has been adopted very widely as a cryptographic method. He arranged the letters in a square and numbered the rows and columns. To use the English alphabet, and merging i and j in a single cell to fit the alphabet into a 5 × 5 square: Each letter may now be represented by two numbers—that of its row and that of its column. Thus e = 15, v = 51. Polybius suggested that these numbers be transmitted by means of torches—one torch in the right hand and five in the left standing for e, for example. This method could signal messages over long distances. But modern cryptographers have found several characteristics of the Polybius square, or “checkerboard,” as it is now commonly called, exceedingly valuable—namely, the conversion of letters to numbers, the reduction in the number of different characters, and the division of a unit into two separately manipulable parts. Polybius’ checkerboard has therefore become very widely used as the basis of a number of systems of encipherment.””
