View source for Tap code

{{Short description|Encoding for text messages}}
{| class="wikitable" align=right style="width:100%; width:200px; text-align:center;"
|+  '''Latin alphabet tap code table'''
!
! 1 !! 2 !! 3 !! 4 !! 5 
|-
! 1
| [[A]] || [[B]] || [[C]]/[[K]] || [[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]]
|}

The ''tap code'', sometimes called the '''knock code''', is a way to encode text messages on a letter-by-letter basis in a very simple way. The message is transmitted using a series of tap sounds, hence its name.<ref>The Handbook Of The SAS And Elite Forces. How The Professionals Fight And Win. Edited by Jon E. Lewis. p.199-Tactics And Techniques, Evasion, Capture And Escape. Robinson Publishing Ltd 1997. ISBN 1-85487-675-9</ref>

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.

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.

To communicate the word "hello", the cipher would be the following (with the pause between each number in a pair being shorter than the pause between letters):

{| class="wikitable"
|-
! H !! E !! L !! L !! O
|-
| {{center|2, 3}}|| {{center|1, 5}}|| {{center|3, 1}}|| {{center|3, 1}}|| {{center|3, 4}}
|-
| colspan = 5 | •• ••• &nbsp; • ••••• &nbsp; ••• • &nbsp; ••• • &nbsp; ••• ••••
|}

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=Reader's Digest Press |location=New York |isbn=0-88349-112-5}}</ref> The tap system simply requires one to know the alphabet and the short sequence "AFLQV" (the initial letter of each row), without memorising the entire grid. For example, if a person hears four knocks, they can think "A... F... L... Q". If after a pause there are three knocks, they think "Q... R... S" to arrive at the letter S.

== History ==
{| class="wikitable" align=right style="width:100%; width:200px; text-align:center;"
| colspan=7|  '''Russian alphabet tap code'''<ref>{{Cite web|url=http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html|title=Far Outliers: Russian Prison Tapping Code|date=April 14, 2005|access-date=August 8, 2014|archive-date=August 15, 2014|archive-url=https://web.archive.org/web/20140815063344/http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html|url-status=live}}</ref>
|-
! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6
|-
! 1
| [[А]] || [[Б]] || [[В]] || [[Г]] || [[Д]] || [[Е]]/[[Ё]]
|-
! 2
| [[Ж]] || [[З]] || [[И]]/[[Й]] || [[К]] || [[Л]] || [[М]]
|-
! 3
| [[Н]] || [[О]] || [[П]] || [[Р]] || [[С]] || [[Т]]
|-
! 4
| [[У]] || [[Ф]] || [[Х]] || [[Ц]] || [[Ч]] || [[Ш]]
|-
! 5
| [[Щ]] ||[[Ъ]]/[[Soft sign|Ь]]||[[Ы]] || [[Э]] || [[Ю]] || [[Я]]
|}

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/americanexperience/features/honor-tap-code/ | title='Return with Honor': The Tap Code | work=[[American Experience]] | publisher=[[Public Broadcasting Service|PBS]] | year=1999 | access-date=2008-04-08 | archive-url=https://web.archive.org/web/20121110134840/https://www.pbs.org/wgbh/amex/honor/sfeature/sf_tap.html | archive-date=2012-11-10 | url-status=live}}</ref><ref>{{Cite book|title=Taps on the Walls: Poems from the Hanoi Hilton|last=Borling|first=John|publisher=Master Wings Publishing|year=2013|isbn=978-0-615-65905-3|location=Priztker Military Library|pages=1–9}}</ref> Harris had heard of the tap code being used by prisoners in World War II<ref name="au">{{cite news | url=http://www.au.af.mil/au/awc/awcgate/awc-pow.htm | title=Vets, Flyers discuss ideology, time in POW camps | author=Staff Sgt. Jason Tudor | publisher=[[Air Force News Service]] | date=1998-03-18 | access-date=2008-04-08 | archive-url=https://web.archive.org/web/20071203095911/http://www.au.af.mil/au/awc/awcgate/awc-pow.htm | archive-date=2007-12-03 | url-status=dead}}</ref> and remembered a [[United States Air Force]] instructor who had discussed it as well.<ref name="pbs"/><ref>{{Cite book|title=Defiant : the POWs who endured Vietnam's most infamous prison, the women who fought for them, and the one who never returned|last=Townley, Alvin.|date = 4 February 2014|isbn=9781250006530|edition=First edition : February 2014|location=New York|oclc=862575088}}</ref>

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>

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.<ref>{{cite web |title=Rough Translation: How 'Anna Karenina' Saved A Somali Inmate's Life |url=https://www.npr.org/2017/09/11/550058353/rough-translation-how-anna-karenina-saved-a-somali-inmates-life |website=[[NPR]] |date=2017-09-11 |archive-url=https://web.archive.org/web/20230626065411/https://www.npr.org/2017/09/11/550058353/rough-translation-how-anna-karenina-saved-a-somali-inmates-life |archive-date=2023-06-26 |url-status=live}}</ref>

== In popular culture ==
{| class="wikitable" align=right style="width:100%; width:200px; text-align:center;"
| colspan=7|  '''''[[Breaking Bad]]'' alphanumeric tap code'''
|-
! !! 1 !! 2 !! 3 !! 4 !! 5 !! 6
|-
! 1
| А || B || C || D || 1 || 2
|-
! 2
| E || F || G || H || 3 || 4
|-
! 3
| I || J || K || L || M || N
|-
! 4
| O || P || Q || R || S || T
|-
! 5
| U || V || W || X || Y || Z
|-
! 6
| 5 || 6 || 7 || 8 || 9 || 0
|}
In the video game ''[[Metal Gear 2: Solid Snake]]'', the tap code is used by Dr. Drago Pettrovich Madnar to communicate to ''[[Solid Snake]]'' through a cell wall.

In Season 2 Episode 2 of ''[[Person of Interest (TV series)|Person of Interest]]'', the tap code is used by Harold Finch to discreetly leave breadcrumbs of his location to John Reese by encoding his location as tap code on a telephone.

In the video game ''[[Her Story (video game)|Her Story]]'', the main characters use the tap code to surreptitiously communicate. 

In Season 2 Episode 14 of ''[[The Flash (2014 TV series)|The Flash]]'', the masked prisoner in Zoom's lair uses the tap code to try to communicate with the others.

In the 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.

In the television show ''[[Breaking Bad]]'', the character [[Hector Salamanca]] uses the version of a tap code shown in this section to communicate after suffering a [[stroke]].

In an ARG for [[Cool Math Games]], (See Game Theory for more information), a tap code was used twice, once in pair with another cypher to solve and discover lore.

==See also==
*[[Morse code]]

==References==
{{Reflist}}

==External links==
* [http://www.braingle.com/brainteasers/codes/tapcode.php Online Tap Code Encoder/Decoder]
* [http://faroutliers.blogspot.com/2005/04/russian-prison-tapping-code.html Russian Prison Tap Codes]
*[http://www.premioceleste.it/opera/ido:260668/ L' Alfabeto Quadrato(Codice a Colpi) - An Degrida] Artist An Degrida's Tap Code artistic illustration.

{{Clear}}
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[[Category:Classical ciphers]]
[[Category:Encodings]]