The origins of this encoding go back to the
Polybius square of
Ancient Greece. Like the "knock code", a
Cyrillic script version is said to have been used by
nihilist prisoners of the
Russian czars. The knock code is featured in
Arthur Koestler's 1941 work
Darkness at Noon.
Kurt Vonnegut's 1952 novel
Player Piano also includes a conversation between prisoners using a form of tap code. The code used in the novel is more primitive and does not make use of the Polybius square (e.g. "P" consists of sixteen taps in a row). 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
Hỏa Lò ("Hanoi Hilton") prison: Captain
Carlyle "Smitty" Harris, Lieutenant
Phillip Butler, Lieutenant Robert Peel, and Lieutenant Commander Robert Shumaker. 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 Vietnam, the tap code became more widely used than Morse; despite messages taking longer to send, the system was easier to learn and could be applied in a wider variety of situations. 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 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. U.S. Navy Rear Admiral
Jeremiah Denton developed a vocal tap code of coughs, sniffs and sneezes. In 1980, a doctor sentenced to life in solitary confinement in
Somalia used tap code to share the entirety of Tolstoy's
Anna Karenina, nearly 2 million letters, via tap code with fellow prisoners. == In popular culture ==