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The Polybius Square Explained
The Polybius square turns each letter into two numbers, a row and a column, using a 5x5 grid. Described by the Greek historian Polybius around 150 BCE as a way to signal letters with torches over distance, it is one of the oldest surviving methods for turning writing into numbers.
Last updated 14 July 2026
How the grid is built
The 26 letters do not divide evenly into a 5x5 grid of 25 cells, so I and J are combined into a single cell. The remaining letters fill the grid in order, five per row, giving every letter a unique row and column from 1 to 5.
Reading a letter's coordinates off the grid takes seconds once you know the layout, which is exactly why Polybius designed it for signalling: a sender only needs to hold up torches in two groups, one for the row and one for the column.
Where the idea resurfaces
The same 5x5 grid became the basis for later systems, most famously tap code, which prisoners of war used to communicate through cell walls by tapping the row and column of each letter instead of writing digits.
Because it needs no key, a Polybius square offers no real secrecy today. Its lasting value is as the simplest possible bridge between letters and numbers, and as a building block other, stronger ciphers were later built on.
Frequently asked questions
How is the Polybius square different from tap code?
Both use a 5x5 grid and a row-column coordinate for each letter, but the classic Polybius square merges I and J while tap code merges C and K, and tap code signals the numbers as taps rather than written digits.
Does the Polybius square use a keyword?
The classic version does not; the grid always follows the standard alphabet order. Some later variants scramble the grid with a keyword, similar to the Playfair cipher, for extra secrecy.