Encode and decode the Polybius square, turning each letter into a pair of digits from a 5×5 grid. I and J share a cell. A classic cipher.
It is a 5 by 5 grid filled with the alphabet, used to turn each letter into the two digits of its row and column. Because the grid has 25 cells and the alphabet has 26 letters, I and J share one cell.
Each letter becomes a two-digit number: the first digit is the row (1 to 5) and the second is the column. For example with the standard grid, H is 23. Decoding pairs the digits back into letters.
On its own it is a simple cipher, but the Polybius square is the foundation of more advanced ciphers like the bifid, trifid and the ADFGVX cipher, and it was historically used for tap codes.