Tap Code / Knock Code

The tap code is a simple way to encode messages. The messages are sent by translating letters into knock sounds. It was used by prisoners in Vietnam, and is sometimes called knock code or Smitty code. It uses a 5x5 square to assign two numbers (row and column) to each letter. Then the sender knocks as many times as each number, and makes pauses between the numbers.

Example

Let's look at the word HELLO. First translate it to numbers using the square (a so called Polybius square):

H  E  L  L  O
23 15 31 31 34
        

Then translate them to dots.

2  3    1 5      3   1  3   1  3   4
.. ...  . .....  ... .  ... .  ... ....

Features

• Tap code has some similarities with Morse code, but only short "beeps" are used. Like Morse code, tap code can be transmitted in various ways and in difficult environments.
• Other names for tap code include knock code and Smitty code.
• A Russian variant exists, where a 5x6 rectangle of cyrillic letters is used instead of the 5x5 square.
• A variant of tap code was featured in Kurt Vonnegut's novel Player Piano.