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The tap code table
The tap code is a cipher, commonly used by prisoners to communicate with one another. The method of communicating is usually by "tapping" either the metal bars or the walls inside the cell, hence its name. It is a very simple cipher, not meant to avoid interception, since the messages are sent in cleartext.
The tap code is based on a 5×5 grid of letters, representing all the letters of the Latin alphabet except K (C is used to represent K). Each letter was communicated by tapping two numbers: the first designated the row (horizontal) and the second designated the column (vertical). The letter "X" was 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 would tap once, pause, and then tap once again.
Or to communicate the word "WATER" the cipher would be the following (the time between each pair of numbers is smaller than the one between two different letters):
..... .. . . .... .... . ..... .... ..
(5,2) (1,1) (4,4) (1,5) (4,2)
W A T E R
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 Morse requires the ability to create two different sounding taps. A Morse novice would also need to keep a cheat sheet until he remembers every letters code, which his captors would likely confiscate. Tap code can be easily decoded in one's head by mentally using the table.