The Polybius Square

Send secrets with fire

Long before phones, a Greek thinker named Polybius found a way to send any message across the hills at night. All he needed was a square of letters and ten torches.

Work through the menu at the top, one stop at a time.

1. The Story: find out who Polybius was and why one fire was not enough.
2. The Square: learn how every letter gets two numbers.
3. Training: get fast at finding letters.
4. Seven Codes: read seven torch messages while the key slowly fades away.
5. Tap Code: hear the square knocked through a wall.
6. Signal Tower: locked until you earn the first five badges. A surprise is waiting there.

One more thing. The torches at the very top of this page are spelling a word, over and over. Come back and read it when you know the code.

Fire on the hills

More than 2,000 years ago, there were no phones and no mail trucks. If an army was coming, how could you warn a city far away?

People used fire. A watcher on a hill lit a big fire. A watcher on the next hill saw it and lit one too. The news jumped from hill to hill, faster than any horse could run.

But a signal fire had one big problem. It could say only one thing, and everyone had to agree on what it meant before it was lit. A fire could say "Danger!" It could not say "Send 50 ships" or "The bridge is broken."

Meet Polybius

Polybius (say it like this: puh-LIB-ee-us) was born in Greece around the year 200 BC. He was a soldier, a traveler, and a writer of history. He wrote a huge set of books called The Histories.

In one of those books, he explained a clever way to send any message with torches. He said two men named Cleoxenus and Democleitus came up with the idea, and that he made it better.

Two groups of torches

Here is how it worked. The letters of the alphabet were written in a square with 5 rows and 5 columns.

The sender had two groups of torches. The torches on the left showed the row. The torches on the right showed the column.

Two torches on the left and three on the right meant row 2, column 3. The watcher on the far hill looked at the square, found that box, and wrote down the letter. Letter by letter, the torches could spell anything.

A square that did not quite fit

Polybius used the Greek alphabet, which has 24 letters. His square had 25 boxes, so one box was left empty.

English has 26 letters. That is one too many for 25 boxes! So in our square, the letters I and J share a box.

What happened next

The torch system was not really a secret. Anyone who knew the square could read the torches. But the big idea, turning letters into numbers, changed codes forever.

Much later, people knocked the numbers on walls to talk in secret. In World War I, an army hid its messages with a code built on a square like this one. Today, computers turn every letter you type into numbers too.

Seven messages from the east tower

This story is made up, but the code is real. A lookout on the east tower is sending torch messages to the city. You are the reader on the city wall.

Decode each word and type it in its box. Watch out. As the night goes on, your key starts to fade. The letters still go in ABC order, so you can work out the missing ones.

The tap code

Here is a strange fact. You do not need torches to use the square. You only need to count.

Knock for the row. Wait a moment. Then knock for the column. Prisoners of war used this trick to talk to each other through walls. It is called the tap code.

The real tap code used a square that was a tiny bit different. It left out K and used C in its place. We will keep using our square so you do not have to learn a new one.

How to listen

1. Get paper and a pencil.
2. Count the first group of knocks. That is the row.
3. After a short pause, count the second group. That is the column.
4. A long pause means a new letter is coming.

No sound on your computer? Watch the circle. It flashes with every knock.