Tap code is a simple way to encode a text message letter by letter. The message is transmitted by a series of taps. It was often used by prison inmates to communicate across locked cells by tapping on the metal bars of prison doors, water pipes, or the walls between cells.
The Tap code is based on a Polybios square and uses a 5×5 grid of letters, with 25 letters of the alphabet. That is, it must be specified which letter is not used.
The receiver only needs to recognize the time interval of each tapper to identify individual letters.
Each letter is transmitted by tapping two numbers:
- the first number determines the row (from top to bottom)
- the second number determines the column (from left to right)
The pause between these two numbers is shorter than the pause between two consecutive letters.