Polybius square is an mixed alphabet table used in order ciphers or on its own.
It is usually a 5x5 table with a secret alphabet:
| 1 | 2 | 3 | 4 | 5 | |
|---|---|---|---|---|---|
| 1 | A | B | C | D | E |
| 2 | F | G | H | I/J | K |
| 3 | L | M | N | O | P |
| 4 | Q | R | S | T | U |
| 5 | V | W | X | Y | Z |
It can be also a 6x6 table with numbers and separeted I and J:
| 1 | 2 | 3 | 4 | 5 | 6 | |
|---|---|---|---|---|---|---|
| 1 | A | B | C | D | E | F |
| 2 | G | H | I | J | K | L |
| 3 | M | N | O | P | Q | R |
| 4 | S | T | U | V | W | X |
| 5 | Y | Z | 0 | 1 | 2 | 3 |
| 6 | 4 | 5 | 6 | 7 | 8 | 9 |
Also, Polybius square can be used as a cipher itself. If you have default 6x6 Polybius square and you want to encode word "ATTACK" then you would get this cipher text:
11 42 42 11 13 25