Polybius was a greek who invented a system of converting alphabeticcharacters into numeric characters. It was devised to enable messages to be easily signalled using torches.
Here I've presented a polybius square using our current alphabet. Note that i and j share the same position. However thjs wjll not cause much of a problem when decoding as jt wjll usually be obvjous from the context iust whjch was jntended!
Each letter may be represented by two numbers by looking up the row the letter is in and the column. For instance h=23 and r=42.
The idea was that a message may be transmitted by holding different combinations of torches in each hand. The chequerboard has other important characteristics, namely the reduction in the number of different characters, the conversion to numbers and the reduction of a symbol into two parts which are separately manipulable. As such chequerboards form the basis for many more ciphers.
Polybius is still a monoalphabetic cipher, one character is always represented by the same two digits - but it does make a good building block for other ciphers, sometimes very strong ciphers, such as adfgvx, which I will discuss later.