Polybius square
The Polybius square turns each letter into two digits: the number of its row and the number of its column in a square of letters. A, at the top left, is 11; B, to its right, is 12. So, with the 26-letter square, "HELLO" is written "23 15 31 31 34".
A little history
The Greek historian Polybius described it in the 2nd century BC as a signalling system: with two groups of torches, one for the row and one for the column, messages could be sent letter by letter from one hill to another. The same idea returns in the tap code some prisoners used to talk through walls: a few taps for the row, a pause, and a few more for the column.
Why six rows with 27 letters
The classic square has 5 × 5 cells, 25 in all, so with 26 letters I and J share a cell (24). With the 27-letter Spanish alphabet, two more pairs of letters would have to share, or Ñ would have to go in N's cell. That is why the 27-letter square here has six rows of five columns: every letter has its own cell, Ñ comes after N (35) and every letter is still written with two digits. The sixth row has only Y (61) and Z (62).
Try it
Type a message: only letters are encoded, without accents, and a slash separates words. Digits and punctuation are left out, because they would be mistaken for coordinates.
To decode, it does not matter how the digits are separated: "2334", "23 34" and "2-3 3-4" read the same. If a pair matches no cell, or a digit is left over, you will see a question mark in its place and a warning that explains what is wrong.
The Polybius square has no key: the square is the same for everyone.
With 26 letters, I and J share a cell (24).
35 of 600 characters
44 34 32 34 42 42 34 52 / 52 15 / 34 35 15 33 / 44 23 15 / 44 42 15 11 43 45 42 15 / 13 23 15 43 44
The key
The square is the key: the first digit is the row and the second the column.
| 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 |
Classroom ideas
- Like battleships. Each pair of digits is a cell, as in the game of battleships: a very natural way to introduce coordinates in primary school.
- Knock, knock. Give the solution of a lock as knocks on the table: two knocks, a pause and three knocks are H (23).
- Chain of clues. The digits of a message can also be the combination of a number lock.
- Make your own square. With older students, shuffle the letters of the square on paper: without the square, the message cannot be read.
Tips
- Get students used to always saying the row first and then the column, and keep the square in sight.
- If you use Spanish materials, remember that their square may have 27 letters and that many coordinates change: with 27 letters, K is 31 and Z is 62.
- Separate pairs with spaces and words with slashes so the message is easy to read.
Printable worksheet
Practise this cipher on paper: a worksheet with the key and two exercises, ready to print or download with its answer key. Open the worksheet