United States prisoners of war during the Vietnam War are most known for having used the Tap Code. It was introduced in June 1965 by four POWs held in the Hoa Lo "Hanoi Hilton" prison: Captain Carlyle "Smitty" Harris, Lieutenant Phillip Butler, Lieutenant Robert Peel, and Lieutenant Commander Robert Shumaker.
The origins of this encoding go back to the Polybius square of Ancient Greece. As the "knock code", a Cyrillic alphabet version is said to have been used by nihilist prisoners of the Russian Czars. The knock code is featured in Arthur Koestler's classic 1941 work Darkness at Noon. Smitty Harris had heard of the tap code being used by prisoners in World War II and remembered a United States Air Force instructor who had discussed it as well.
In the Tap Code, 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". Unlike Morse code, the tap code depended on actual taps, instead of the frequency and duration of each tap, to communicate a letter.
The Tap Code is outlined in the table below:
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.
In Vietnam, the tap code became a very successful way for otherwise isolated prisoners to communicate. POWs would use the tap code in order to communicate to each other between cells in a way which the guards would be unable to pick up on. They used it to communicate everything from what questions interrogators were asking (in order for everyone to stay consistent with a deceptive or bogus story), to who was hurt and needed others to donate meager food rations. It was easy to teach and newly arrived prisoners became fluent in it within a few days. It was even used when prisoners were sitting next to each other but not allowed to talk, by tapping on anothers' thigh. By overcoming isolation with the tap code, prisoners were able to maintain a chain of command and keep up morale.