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The Tap Code is a cipher, commonly used by prisoners in jail 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.

It was reportedly invented in June 1965 by four POWs imprisoned in Hoa Lo, Vietnam: Captain Carlyle ("Smitty") Harris, Lieutenant Phillip Butler, Lieutenant Robert Peel and Lieutenant Commander Robert Shumaker. Harris remembered an Air Force instructor who had shown him a cipher based on a Polybius square, as shown on the graph below. 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 and the letter "C" replaced the letter "K".

The Tap Code is featured in Arthur Koestler's classic work Darkness at Noon, which was published in 1941. The prisoners refer to it as the knock code, but the technique is still the same.

The Tap Code is outlined in the table below:

	1	2	3	4	5
1	A	B	C	D	E
2	F	G	H	I	J
3	L	M	N	O	P
4	Q	R	S	T	U
5	V	W	X	Y	Z

For example, to specify the letter "A", you would tap roughly the following: . .

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, most prisoners devised abbreviations and acronyms for common items or phrases, such as "GN" for Good Night, or "GBU" for God Bless You.

There is however the possibility to do with less taps. First one can arrange the letters in a diagonal fashion. The code shown below never needs more than 8 taps per letter opposed to letters with 9 and 10 taps in the "standard" tap code. Second you can exploit letter frequency as in Morse code. Obviously E should be the code with two taps, not A.

	1	2	3	4	5	6	7
1	E	A	N	D	W	V	?
2	T	I	R	M	B	.
3	O	H	U	P	Z
4	S	C	Y	Q
5	L	G	X
6	F	J
7	K

Here is the word WATER with the standard tap code and the modified version.

..... ..  . .  .... ....  . .....  .... ..
..... .  .. .  . ..  . .  ... ..

• Knock three times (disambiguation)

• The Tap Code
• Online Tap Code Encoder/Decoder
• Koestler, Arthur, Darkness at Noon (1941). Translated by Daphne Hardy. See page 19 of the Bantam Publishing paperback, 1981 printing for more info.