Tap code translator

Type on the left and the taps appear on the right. Each letter comes out as two runs of dots with a small gap between them, and a wider gap before the next letter.

Write taps as switches the output to bare numbers, so C reads 1,3 instead of a row of dots. Both notations decode, so you can paste either one back in.

The alphabet sits in a 5x5 grid. The first run of taps counts down to the row, the second counts across to the column. A is 1 and 1, so it's one tap, pause, one tap. Z is five and five.

K has no cell. Twenty-six letters don't fit in 25 squares, and dropping K costs almost nothing in English, so a K is tapped as C and the reader works it out. This is the one place the tap code differs from a plain Polybius square, which merges I and J instead.

Four American prisoners taught it to each other in the Hoa Lo Prison in Hanoi in June 1965. Carlyle Harris remembered it from an Air Force instructor who had shown him a code built on a 5x5 grid. Harris passed it to Phillip Butler, Robert Peel and Robert Shumaker, and it spread through the camp.

Prisoners were held in isolation and punished for communicating. The code needed no paper, no light and no shared language beyond the grid. Men tapped it through walls, coughed it and swept it.

James Stockdale, the senior naval officer among them, wrote in In Love and War that the tapping "ceased to be just an exchange of letters and words; it became conversation."

Morse code solves the same problem with one channel instead of two, at the cost of a code book you have to memorise. Tap code needs only a 5x5 grid you can reconstruct from scratch.

A1Z26 also turns letters into numbers, but it counts along the alphabet rather than across a grid.