An early character encoding system that arranges the alphabet into a grid, typically five by five. Each letter is represented by its row and column numbers, converting text into pairs of digits. Originally designed for optical telegraphy using pairs of torches, it forms the basis for many later substitution ciphers and tap codes used in low-bandwidth signaling.
Heard on the job
The prisoners communicated through the pipes using a Polybius square, tapping twice for row two and three times for column three to send an H.
Insider tip
In the standard English five-by-five grid, the letters I and J usually share a single cell to make the 26-letter alphabet fit into 25 spaces.