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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.[1]

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

[edit]

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.[2]

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):

H E L L O
2, 3
1, 5
3, 1
3, 1
3, 4
•• •••   • •••••   ••• •   ••• •   ••• ••••

The letter "X" is used to break up sentences, and "K" for acknowledgements.

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.[2]

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.[3] 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

[edit]
Russian alphabet tap code[4]
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/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 prisoners of the Russian czars.[5] The knock code is featured in Arthur Koestler's 1941 work Darkness at Noon.[6] 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.[2][7] Harris had heard of the tap code being used by prisoners in World War II[8] and remembered a United States Air Force instructor who had discussed it as well.[2][9]

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.[3] Tap codes proved to be a very successful[10] way for otherwise isolated prisoners to communicate.[8][11] 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.[12] 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.[13][14] It was even used when prisoners were sitting next to each other but not allowed to talk, by tapping on another's thigh.[14] U.S. Navy Rear Admiral Jeremiah Denton developed a vocal tap code of coughs, sniffs and sneezes.[3] By overcoming isolation with the tap code, prisoners were said to be able to maintain a chain of command and keep up morale.[8][15]

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.[16]

[edit]

In the 1990 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 in 2012, 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 2015 video game Her Story, the main characters use the tap code to surreptitiously communicate.

In Season 2 Episode 14 of 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

[edit]

References

[edit]
[edit]
Revisions and contributorsEdit on WikipediaRead on Wikipedia

Tap code

View on Grokipedia
from Grokipedia
Tap code is a rudimentary signaling system for encoding and transmitting alphabetic messages via sequences of taps, typically on cell walls, pipes, or bars, employing a 5×5 grid () that maps 25 letters of the Latin alphabet—usually combining C and K or I and J—to row and column positions, with each letter signaled by the corresponding pair of tap counts separated by a brief pause. Introduced to American prisoners of war in in June 1965 by Captain Carlyle "Smitty" Harris and Lieutenants Phillip , Robert , and Robert Shumaker while confined in Hỏa Lò Prison (the "Hanoi Hilton"), the code enabled isolated inmates to exchange personal messages, intelligence, and directives for resisting interrogations and maintaining military structure. By August 1965, it had proliferated among most POWs despite detection risks, transforming into a network of subtle communication that fostered unity, humor, and defiance against captors' psychological tactics. Its adoption underscored the code's ease of memorization and execution—requiring no tools beyond rhythmic tapping—allowing even severely tortured prisoners to participate, with pauses delineating letters and an "X" (5-2 taps) serving as punctuation to structure transmissions.

System and Mechanics

The Polybius-Inspired Grid

The tap code utilizes a 5×5 grid, adapted from the ancient , to assign each letter of the a unique pair of coordinates corresponding to row and column positions. This structure enables encoding by translating letters into sequences of taps, where the first group of taps indicates the row and the second the column, separated by a pause. The grid excludes the letter K—typically represented by C in decoding when needed—to fit 25 letters into the matrix, allowing I and J to occupy distinct positions. The standard arrangement follows in row-major sequence:
12345
1ABCDE
2FGHIJ
3LMNOP
4QRSTU
5VWXYZ
This layout traces its inspiration to the , documented by the Greek historian (c. 200–118 BC) in his Histories as a method for long-distance signaling via torch positions, where coordinates replaced letters to facilitate transmission by observers. In tap code applications, the grid's numerical coordinates convert to auditory or tactile signals: for instance, "A" (row 1, column 1) is conveyed as a single tap, a pause, then another single tap; "R" (row 4, column 2) as four taps, pause, two taps. Variations in grid filling or letter omission exist across implementations, but the alphabetical row-wise order predominates in documented prison and military uses for its memorability and simplicity.

Encoding and Transmission Methods

Tap code encodes letters using a 5×5 grid where each letter corresponds to a unique row and column coordinate, with the letters I and J typically combined in a single cell. To transmit a letter, the sender produces a series of taps equal to the row number (1 to 5 quick taps), followed by a brief pause, then another series equal to the column number, with a longer pause signaling the end of the letter. For instance, the letter "A" at position (1,1) requires one tap, pause, one tap; "S" at (4,3) requires four taps, pause, three taps. Transmission occurs through physical on rigid surfaces to propagate vibrations or sounds to adjacent cells, commonly using metal bars, , or concrete walls in or settings. Senders often employ knuckles, rings, or small objects to generate distinct, audible knocks, ensuring the rhythm distinguishes intra-number taps (minimal pause), inter-coordinate pauses (short), and inter-letter pauses (long). This method relies on auditory or tactile reception by the recipient, who listens or feels the vibrations through shared structural elements.

Decoding and Error Correction

Decoding in tap code requires the recipient to interpret sequences of taps as coordinates on a 5x5 grid, which maps the 25 letters of the (typically combining C and K in one cell, omitting J). The sender transmits each letter by tapping a number of times equal to the row (1 to 5 taps), pausing briefly to signal the transition, then tapping the column number (1 to 5 taps), followed by a longer pause or silence to delineate the letter. The recipient counts the taps in the first group to identify the row, awaits the pause, counts the second group for the column, and cross-references the intersection on the memorized grid to retrieve the letter; this process repeats letter-by-letter for the message, with word separations often indicated by longer silences or contextual spacing. Error correction in tap code lacks formal parity checks or redundancy schemes typical of engineered codes, relying instead on informal, interactive protocols adapted to the medium's constraints. Senders signal transmission errors—such as miscounts or interruptions—by producing a burst of rapid, irregular taps (e.g., several quick taps in succession) to alert the recipient, after which the sender restarts the affected letter or . Recipients, upon detecting ambiguities through or unfamiliar letter combinations, can request clarification by tapping back a predefined signal (e.g., a repeat query like "" for "repeat") or echoing the received for verification, enabling bidirectional adjustment in adjacent cells. In prolonged uses, such as among U.S. POWs in from 1965 onward, integrity was further maintained through deliberate repetition of critical content, slow pacing to minimize miscounts, and reliance on shared linguistic to infer and correct minor distortions without explicit codes.

Historical Development

Ancient and Pre-Modern Origins

The , the encoding foundation of tap code, originated in during the Hellenistic period, circa 200–150 BCE. Credited to the scholars Cleoxenus and Democleitus, it was documented by the historian in Book X of his Histories, where he described its use for coordinating military signals over long distances. The system organized the Greek alphabet (excluding certain letters or combining them) into a 5x5 grid, with letters identified by their row and column positions—typically signaled visually by holding torches in corresponding spots on a pair of frames, one for rows and one for columns. This allowed sentinels on hilltops to transmit messages letter by letter, enabling rapid communication across battlefields or cities without verbal shouts or messengers, as demonstrated in Polybius's account of its potential to convey complex intelligence like enemy movements. Originally designed for optical rather than auditory transmission, the emphasized discrete, coordinate-based encoding to minimize errors in noisy or distant environments. Polybius highlighted its efficiency for naval and land forces, noting that with practiced operators, it could rival the speed of while resisting interception by non-equipped observers. Archaeological and textual , including references in later Roman adaptations, confirms its practical deployment in Greek and Hellenistic military contexts, predating similar coordinate systems by over two millennia. However, no contemporary sources indicate its use for tactile or methods in antiquity; such innovations required confined, silent settings absent from ancient descriptions. Pre-modern adaptations of grid-based codes remained sparse and undocumented until the , with no verified instances of tapping on walls or pipes prior to industrialized incarceration systems. Early prisoner communications often relied on simpler sequential knocks (e.g., A as one tap, B as two), as reported in accounts from European gaols, but these lacked the grid's positional efficiency. The square's revival for covert signaling appears in military cryptography during conflicts like the Boer War (1899–1902), where British forces employed it for basic , though again primarily in written or visual forms rather than percussive. This historical gap underscores that while the algorithmic core traces to antiquity, tap code's practical auditory form crystallized in 20th-century captivity, adapting ancient principles to modern isolation.

Adoption in Prison Environments

Tap code, also known as knock code, emerged as a covert communication method in settings where verbal interaction was prohibited or severely restricted, such as or high-security isolation units. Inmates transmit messages by tapping on walls, pipes, metal bars, or floors, leveraging the acoustic properties of infrastructure to propagate sounds between cells. This system allows for letter-by-letter encoding without visual or spoken cues, making it suitable for environments enforcing silence to prevent coordination or morale-building among prisoners. Its adoption stems from the practical need for and in dehumanizing conditions, predating formalized applications and rooted in the simplicity of the 5x5 grid adapted for auditory signals. Documented instances of tap code use in civilian prisons date to the , including among Russian political prisoners who employed similar knocking systems to share intelligence and maintain solidarity under tsarist repression. In modern examples, Somali physician Adan Abokor utilized a knock-based code during his imprisonment in the 1980s under the regime; isolated in , he tapped out the entire text of Leo Tolstoy's —approximately 350,000 words—to sustain the of a neighboring inmate on the verge of , demonstrating the code's role in . Such adaptations highlight tap code's versatility in non-Western penal systems, where guards often overlooked rhythmic tapping as mere noise rather than structured language. In Western prisons, including U.S. facilities, tap code persists in supermax and administrative segregation units, where in adjacent cells exchange names, warnings, or plans despite electronic focused on verbal or visual signals. Adoption proliferates organically through inmate subcultures, as the code requires no tools beyond body or improvised objects like cups, and its low-tech nature evades detection in analog-heavy older prisons. However, prison administrations have countered it with , irregular cell layouts, and disciplinary measures against detected tapping, limiting its reliability in contemporary high-tech facilities.

Prominent Use in 20th-Century Military Captivity

Tap code achieved its most widespread and documented use among U.S. prisoners of war in North Vietnamese captivity during the , enabling covert inter-cell communication in facilities like , dubbed the "Hanoi Hilton." U.S. Air Force Captain Carlyle A. "Smitty" Harris, captured after his F-105 Thunderchief was shot down on April 4, 1965, during a strike on the , introduced the system shortly after arrival. Harris had memorized the 5x5 grid from a pre-war Air Force instructor who described its application by POWs tapping on shared water pipes. Prisoners encoded letters by tapping row and column numbers sequentially—pausing between pairs—with the grid omitting J and combining it with I to fit 25 characters. Harris covertly taught the code to early arrivals like Navy Lieutenant Commander Shumaker, who disseminated it further despite risks of detection and . By August 1965, tapping echoed through walls and across cells, allowing transmission of names, ranks, shoot-down dates, and resistance guidelines. This network sustained morale amid isolation, torture, and propaganda efforts, as messages relayed compliance standards like the "four big things" to avoid (no false confessions, no harming fellow POWs, no accepting , no fraternizing with captors). Over 2,871 days for Harris until release on February 12, 1973, the linked hundreds of POWs across camps, preserving command structure and unity—evident when synchronized Christmas carol taps defied guards. Its efficacy stemmed from simplicity and adaptability, evolving from WWII precedents but scaled systematically in to counter enforced silence.

Applications and Contexts

Communication in Incarceration

Tap code, also referred to as knock code, enables incarcerated individuals to transmit messages between cells by tapping sequences on shared walls, pipes, or bars, circumventing restrictions on verbal or visual communication. Each letter is represented by a pair of taps corresponding to its position in a 5-by-5 grid of the alphabet, with pauses distinguishing row from column counts, allowing slow but reliable exchange of full sentences over time. This technique persists in solitary confinement or high-security settings where guards monitor overt interactions, as isolated taps blend with ambient prison noises like footsteps or clanging doors. Historical records document its use among political prisoners in Tsarist Russia, where nihilists adapted a Cyrillic variant to convey revolutionary plans and personal messages through thick cell walls, predating its documented military applications. In the Soviet era, inmates in facilities like Moscow's Lefortovo Prison employed similar knocking systems; American prisoner Alexander Dolgun, held from 1975 to 1978, learned and used it to decode messages by associating taps with letters via memorized aids like matchstick grids. These codes facilitated morale maintenance, information sharing on guard routines, and subtle resistance, with users developing protocols like rhythmic pauses to confirm receipt and minimize detection risks. In contemporary civilian prisons, tap code variants aid in coordinating illicit activities, relaying warnings, or sustaining social bonds despite isolation protocols; for instance, systems in U.S. facilities involve pipes to signal arrivals or threats, though wardens counter with or surveillance. Its simplicity requires no tools beyond body or improvised objects like cups, making it adaptable across languages via grid modifications, but transmission speed—often 1-2 letters per minute—limits it to essential, non-urgent exchanges. Despite occasional guard awareness, the code's persistence underscores its role in preserving agency amid enforced silence, with documented instances in supermax units where alternatives like notes are infeasible.

Military and POW Scenarios

Tap code gained prominence as a covert communication method among American prisoners of war (POWs) during the , enabling isolated captives to exchange information despite and guard surveillance. In the , derisively called the "Hanoi Hilton" by inmates, U.S. Captain Carlyle "Smitty" Harris, shot down over on April 4, 1965, introduced the system to fellow POWs after recalling it from survival training. Harris shared the code with Phillip A. Butler, Lieutenant William M. "Bill" Lawrence, and Lieutenant in June 1965, marking its initial dissemination among the early shoot-downs held there. By August 1965, the method had proliferated across the facility, with POWs tapping sequences on cell walls, metal beds, or plumbing pipes to convey letters via the 5x5 grid, where the first tap series indicated the row and the second the column (combining C and K in position 1-3). This allowed transmission of critical updates, such as resistance strategies, names of new arrivals, and morale-boosting messages, fostering a sense of unity among the approximately 500 American POWs eventually held in prisons, predominantly downed aviators. Harris himself endured 2,871 days in captivity, during which the code facilitated underground coordination that undermined captor efforts to break prisoner cohesion. Guards periodically detected the tapping—audible as rhythmic knocks—and responded with , including beatings and extended isolation, yet the system's simplicity and deniability ensured its persistence as a psychological lifeline. During events like the on July 6, 1966, where 52 POWs were paraded through streets before crowds, pre-march tap code exchanges reinforced the "" adherence, emphasizing name, rank, service number, and date of birth only. Beyond Hỏa Lò, the code spread to other North Vietnamese camps, sustaining communication networks that preserved operational security and mental resilience until the facilitated releases starting in 1973. Its efficacy stemmed from low-tech execution requiring no tools beyond a knuckle or fingernail, making it adaptable to the austere, monitored conditions of military detention.

Survival and Non-Institutional Uses

Tap code has been recommended in as a low-tech method for communicating through physical barriers or over short distances in emergencies, such as being trapped in collapsed structures, caves, or during disasters like earthquakes or accidents. By tapping out the row and column numbers corresponding to letters on a 5x5 grid, individuals can spell messages on available surfaces like walls, pipes, or the ground, enabling coordination with rescuers or companions without voice or equipment. This approach requires only basic memorization of the grid, making it accessible for rapid deployment in high-stress or urban scenarios where verbal signals fail due to noise, injury, or separation. In wilderness contexts, extends beyond auditory knocks to improvised signals, such as rhythmic strikes with sticks on trees or rocks to alert distant group members of location or distress, serving as a simpler alternative to that avoids the need for distinguishing dots from dashes. guides emphasize its utility for pre-planned team signaling during hikes, hunts, or expeditions, where participants agree on the beforehand to transmit coordinates, warnings, or requests for aid. For example, it can convey essential phrases like "HELP" (4-3, 2-3, 2-4, 3-4) through patterned taps, potentially critical in or remote terrain where visibility is limited. Non-institutional adaptations include its promotion in civilian preparedness for scenarios like home invasions, kidnappings, or improvised situations, where subtle wall-tapping allows discreet message relay between separated victims. Additionally, some response training incorporates tap code for aiding those with temporary communication impairments, such as in entrapments or floods, prioritizing its error-resistant paired-tap structure over more complex systems. While primarily auditory, variations using light flashes or drum beats have been suggested for open environments, though efficacy diminishes beyond close range without amplification.

Strengths and Limitations

Operational Advantages

Tap code facilitates communication across physical barriers, such as cell walls or pipes, enabling isolated individuals to exchange messages without line-of-sight or verbal interaction, which is particularly advantageous in high-security incarceration where guards restrict direct contact. In the Hoa Lo Prison during the , U.S. POWs utilized taps on shared structures to propagate signals discreetly, allowing coordination among cells separated by despite protocols. This acoustic transmission method leverages vibrations in solid materials, reducing the need for proximity and minimizing visual cues that could betray the activity to observers. The system's design, based on a 5x5 grid assigning letters A-I to rows and columns (with C/K sharing position 2-3 and J omitted or merged with I), promotes ease of encoding and decoding under , as operators need only count taps for row and column positions separated by a pause, without requiring external aids like paper or instruments. POWs in , starting from Navy Roberson's introduction in August 1965, rapidly disseminated the code through initial verbal teaching, enabling widespread adoption by over 200 captives by late 1965 for relaying personal updates, intelligence, and morale-boosting affirmations. Its mnemonic simplicity—tied to the familiar alphabetic sequence—allowed quick mastery, even by personnel fatigued from interrogation or injury, contrasting with more complex ciphers that demand prolonged study. Operationally, tap code offers resilience against interception, as soft taps can mimic incidental noises like shifting restraints or coughing, evading detection unless captors maintain constant auditory surveillance, which proved impractical in understaffed facilities like Hỏa Lò. During punitive phases from 1967 to 1969, when overt signaling risked torture—such as three days of beatings—prisoners defaulted to this method for 90% of inter-cell exchanges, sustaining operational continuity for resistance efforts like synchronized protests. The protocol's built-in pauses and repeatable sequences further enhance error detection, permitting clarification taps to resolve ambiguities from distance-attenuated signals or ambient interference, thereby upholding message fidelity in austere conditions.

Practical Drawbacks and Criticisms

Tap code's primary practical limitation is its low transmission speed, typically achieving around 10 words per minute under optimal conditions with short intervals between taps, far slower than verbal speech or even equivalents adapted for tapping. This slowness arises from the need to encode each letter via two sequences of 1 to 5 taps separated by a pause, requiring up to 10 taps for letters like , which demands precise counting and rhythm to avoid errors or signal blending. Prolonged use for extended messages becomes tedious, as the repetitive tapping fatigues the sender and risks misinterpretation without strict timing discipline. The code mandates memorization of a 5x5 grid (omitting K or combining I/J), imposing a that, while simpler than Morse for beginners in short bursts, hinders rapid adoption among uninitiated users and limits its utility in scenarios. Its reliance on auditory transmission through walls or pipes restricts range to adjacent cells, rendering it ineffective over distances or in isolated confinement without conductive mediums, and vulnerable to environmental noise that obscures faint taps. Detectability poses a security risk, as rhythmic patterns of taps can mimic construction noise but accumulate into suspicious volumes, as observed in Vietnam War POW camps where widespread use made facilities audibly active like workshops, potentially alerting guards to coordinated activity. Although rarely intercepted directly due to its simplicity masking intent, guards who learned the system could decode messages, as occurred when authorities understood related signals in some camps, leading to separations or punishments. The grid's exclusion of numbers, , and full variants further constrains expressiveness, necessitating workarounds that extend transmission time and introduce , making it unsuitable for precise or urgent data like coordinates or timestamps without predefined adaptations. Overall, while effective for morale-boosting brevity in , these constraints render tap code inefficient for substantive or high-volume exchanges compared to visual or electronic alternatives.

Legacy and Representations

Influence on Modern Communication Techniques

The tap code's grid-based encoding has informed contemporary assistive communication devices and interfaces designed for individuals with motor impairments, where users input text via sequential taps on simplified grids rather than precise key presses. Researchers have adapted prison-derived knock codes into text entry systems that prioritize ease of use and minimal physical effort, enabling non-verbal signaling through vibrations or knocks detectable by sensors. For example, Russian tap-code variants have been explored for communication tools, allowing users to convey letters by tapping anywhere on a device surface without targeting specific areas, thus accommodating tremors or reduced mobility. Similar decoding approaches leverage to interpret irregular tap patterns in real-time, enhancing for those with severe communication disabilities. In and , tap code principles persist as a foundational technique for low-bandwidth, covert signaling in austere environments lacking electronic aids. U.S. forces integrate tap code into resiliency programs, service members to encode and decode messages via wall or pipe taps to simulate POW isolation and foster mental endurance, as demonstrated in training sessions referencing Vietnam-era applications. This extends to broader curricula, where the method's reliance on auditory cues alone—without visual or electronic dependencies—serves as a model for improvised communication during capture or scenarios, emphasizing memorization of the 5x5 for rapid deployment. The code's emphasis on discrete, deniable signals has parallels in modern pattern-based authentication, such as knock code unlocks on mobile devices, which require tapping grid positions in sequence to access functions, mirroring the tap code's positional encoding while adding layers of user-defined complexity for security. However, these commercial implementations prioritize brevity over full alphabetic transmission, adapting the core idea for everyday frictionless interaction rather than prolonged clandestine messaging. Overall, tap code's legacy underscores the value of mnemonic, resource-light protocols in bridging gaps where advanced technology fails, influencing resilient design in both human-centered and tactical communication frameworks.

Depictions in Media and Culture

In the 2021 action thriller film , directed by , trapped miners in a collapsed Canadian diamond mine employ tap code by knocking on metal pipes to signal rescuers and convey critical information about their survival and the cause of the disaster. This portrayal underscores the code's utility in high-stakes, isolated environments beyond traditional incarceration, drawing on its historical association with covert signaling. Tap code appears in literary works depicting imprisonment and resistance. In Arthur Koestler's 1940 novel , the protagonist Nikolai Rubashov, confined in a , uses a rudimentary knock code to exchange messages with an adjacent inmate, facilitating subtle coordination amid and isolation. The technique highlights themes of human endurance and clandestine defiance against authoritarian control, predating the code's widespread adoption in mid-20th-century military contexts. Non-fictional accounts have also influenced cultural representations, as seen in memoirs and documentaries. The 2019 book Tap Code by Carlyle "Smitty" Harris details the code's invention and use among U.S. POWs in Vietnam's ( Hilton), inspiring adaptations in educational media and veteran testimonies that emphasize its role in maintaining morale and command structure. Similarly, PBS's series features segments on the tap code in episodes about POWs, illustrating its transmission via taps on cell walls to share intelligence and resist captors. These depictions reinforce the code's legacy as a symbol of ingenuity in adversity, though fictional uses often simplify its 5x5 grid for dramatic effect.

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  42. https://swling.com/blog/2022/06/kostas-explores-an-alternative-to-morse-code-used-in-prisons-with-roots-in-ancient-greece/
  43. https://dl.acm.org/doi/10.1145/3019943.3019949
  44. https://www.researchgate.net/publication/371523569_An_Efficient_Knock_Code_Decoding_Approach_to_assist_the_People_with_Communication_Impairments
  45. https://www.dvidshub.net/video/592405/whats-your-tap-code
  46. https://www.usenix.org/system/files/soups2020-samuel.pdf
  47. https://subslikescript.com/movie/The_Ice_Road-3758814
  48. http://www.multiwingspan.co.uk/cipher.php?page=tap
  49. https://shelf-awareness.com/readers/2020-01-31/rediscover:_darkness_at_noon.html
  50. https://www.zondervan.com/p/tap-code-epic-survival-tale-vietnam-pow-secret-code-changed-everything/
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