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{{Short description|Encoding for text messages}}
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The
The tap code has been commonly used by prisoners to communicate with each other. The method of communicating is usually by tapping either the metal bars, pipes or the walls inside a cell.
== Design ==
The tap code is based on a [[Polybius square]] using a 5×5 grid of letters representing all the letters of the [[Latin alphabet]], except for K, which is represented by C.<ref name="pbs"/>
Each letter is communicated by tapping two numbers, the first designating the row and the second (after a pause) designating the column. For example, to specify the letter "B", one taps once, pauses, and then taps twice. The listener only needs to discriminate the timing of the taps to isolate letters.
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The letter "X" is used to break up sentences, and "K" for acknowledgements. <!--What does it mean by acknowledgments? Please elaborate-->
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''.<ref name="pbs"/>
By comparison, despite its messages being shorter, [[Morse code]] is harder to send by tapping or banging. Its short and long signals can be improvised as taps and thumps, or short and long whistles or scraping sounds, but tap codes are simpler to learn and can be used in a wider variety of situations.<ref name="denton">{{cite book |last1=Denton |first1=Jeremiah A. |last2=Brandt |first2=Ed |title=[[When Hell Was in Session]] |date=1976 |publisher=
== History ==
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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 movement|nihilist]] prisoners of the [[Russian czars]].<ref>[[David Kahn (writer)|David Kahn]], ''The Codebreakers – The Story of Secret Writing''. 1967. {{ISBN|978-0-684-83130-5}}.</ref> The knock code is featured in [[Arthur Koestler]]'s 1941 work ''[[Darkness at Noon]]''.<ref>Koestler, Arthur, ''Darkness at Noon'' (1941). Translated by Daphne Hardy. See page 19 of the Bantam Publishing paperback, 1981 printing for more info.</ref> [[Kurt Vonnegut]]'s 1952 novel ''[[Player Piano (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 [[Prisoner of war|POWs]] held in the [[Hỏa Lò Prison|Hỏa Lò ("Hanoi Hilton") prison]]: Captain [[Carlyle "Smitty" Harris]], Lieutenant [[Phillip N. Butler|Phillip Butler]], Lieutenant Robert Peel, and Lieutenant Commander Robert Shumaker.<ref name="pbs">{{cite web | url=https://www.pbs.org/wgbh/
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.<ref name="denton"/> Tap codes proved to be a very successful<ref>{{Cite journal|last=Peterson|first=Gordon I|last2=Taylor|first2=David C|date=March 2016|title=Intelligence Support to Communications with US POW's in Vietnam|url=https://www.cia.gov/library/center-for-the-study-of-intelligence|journal=Studies in Intelligence|publisher=Center for the Study of Intelligence Publications|volume=60|pages=1–15|access-date=2019-05-23|archive-url=https://web.archive.org/web/20190508044337/https://www.cia.gov/library/center-for-the-study-of-intelligence|archive-date=2019-05-08|url-status=dead}}</ref> way for otherwise isolated prisoners to communicate.<ref name="au"/><ref>{{Cite book|title=Lessons from the Hanoi Hilton: Six Characteristics of High Performance Teams|last=Fretwell|first=Peter|last2=Kiland|first2=Taylor Baldwin|publisher=Naval Institute Press|year=2013|isbn=978-1-61251-217-4|___location=Annapolis, Maryland|pages=63–67}}</ref> 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.<ref>{{Cite book|title=Two Souls Indivisible: The Friendship that saved two POW's in Vietnam|last=Hirsch|first=James S|publisher=Houghton-Mifflin|year=2004|isbn=0618273484|___location=New York|pages=[https://archive.org/details/twosoulsindivisi00hirs/page/161 161-162]|url-access=registration|url=https://archive.org/details/twosoulsindivisi00hirs/page/161}}</ref> 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.<ref>{{cite book | last=McCain | first=John | author-link=John McCain |author2=Mark Salter |author2-link=Mark Salter | title=Faith of My Fathers | title-link=Faith of My Fathers | publisher=[[Random House]] | year=1999 | isbn=0-375-50191-6|pages=[https://archive.org/details/faithofmyfathersmcc00mcca/page/211 211]–12}}</ref><ref name="brace"/> It was even used when prisoners were sitting next to each other but not allowed to talk, by tapping on another's thigh.<ref name="brace">{{cite book | last=Brace |first=Ernest C. |author-link=Ernest C. Brace | title=A Code to Keep: The true story of America's longest held civilian prisoner of war in Vietnam | publisher=[[St. Martin's Press]] | year=1988 | isbn=0-7090-3560-8|pages= 171–72, 187–88}}</ref> U.S. Navy Rear Admiral [[Jeremiah Denton]] developed a vocal tap code of coughs, sniffs and sneezes.<ref name="denton"/> By overcoming isolation with the tap code, prisoners were said to be able to maintain a [[chain of command]] and keep up morale.<ref name="au"/><ref>{{Cite journal|last=Naughton|first=Robert J|date=1975|title=Motivational Factors of American Prisoners of War Held by the Democratic Republic of Vietnam|url=https://digital-commons.usnwc.edu/nwc-review/vol28/iss1/2|journal=Naval War College Review|volume=28|access-date=2019-05-04|archive-url=https://web.archive.org/web/20190504141451/https://digital-commons.usnwc.edu/nwc-review/vol28/iss1/2/|archive-date=2019-05-04|url-status=live}}</ref>
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== In popular culture ==
In the 1990
In Season 2 Episode 2 of ''[[Person of Interest (TV series)|Person of Interest]]'' in 2012, the tap code is used by Harold Finch to
In the
In Season 2 Episode 14 of ''[[The Flash (2014 TV series)|The Flash]]'' in 2016, the masked prisoner in Zoom's lair uses the tap code to try to communicate with the others.
In the 2021 film ''[[The Ice Road]]'', the tap code is used on a metal pipe conduit by trapped miners to communicate with executives of the mining company.
==See also==
*[[Morse code]]
==References==
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*[http://www.premioceleste.it/opera/ido:260668/ L' Alfabeto Quadrato(Codice a Colpi) - An Degrida] Artist An Degrida's Tap Code artistic illustration.
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