The Polybius Fire Grid

In ancient Greece, a historian named Polybius documented a brilliantly simple way to send messages across miles of empty space. This system didn't need a messenger on horseback. It just needed a mountaintop, a sharp-eyed scout, and two fistfuls of blazing torches.

The trick is a five-by-five grid, like a checkerboard. You number the five rows down the side, and the five columns across the top, then fill the boxes with the alphabet. To send a letter, you just find its box and transmit its row and column numbers.

Instead of ink, ancient soldiers used fire. A scout held up torches on his left side for the row number, and torches on his right for the column number. By flashing these number pairs, they could spell out entire warnings from mountain to mountain.

Centuries later, this exact same mathematical grid helped captured soldiers survive. Prisoners of war realized they didn't need torches to send number pairs. They just needed a wall. They tapped out the row and column numbers, knocking secret messages right under their guards' noses.