Tap Code - Design

Design

The tap code is based on a Polybius square, a 5×5 grid of letters representing all the letters of the Latin alphabet, except for K, which is represented by C. Each letter is communicated by tapping two numbers: the first designating the row (horizontally) and the second designating the column (vertically). The letter "X" is used to break up sentences. The tap code requires the listener to only discriminate the timing of the taps to isolate letters. For example, to specify the letter "A", one taps once, pauses, and then taps once again.

Or to communicate the word "water", the cipher would be the following (the pause between each number in a pair is smaller than the pause between one pair and the next):

Because of the difficulty and length of time required for specifying a single letter, prisoners often devise abbreviations and acronyms for common items or phrases, such as "GN" for Good night, or "GBU" for God bless you.

By comparison, Morse code is harder to send by tapping or banging because it requires the ability to create two differently sounding taps (representing the dits and dahs of Morse code). A Morse code novice would also need to keep a "cheat sheet" until he or she remembers every letter's code, which the captors would likely confiscate. Tap code can be more easily decoded in one's head by mentally using the table.