A character is only able to communicate through a limited set of responses, but tries to convey a lot more information than their medium provides. The title refers to the most extreme version, where the character has only one possible response they can give.
In one standard version of this, the "character" is a spirit at a (generally faked) Spooky Séance, and all they can do is rap (usually because it's easy for whoever's faking the seance to make a rapping sound discreetly under the table). They then end up rapping once for yes, twice for no, and then spelling out a message by having the participants in the seance run through the alphabet, to be stopped by a rap when they reach the next letter in the message.
A common joke is for two raps to be misinterpreted as "yes, yes". Another (often in the seance variant) is for a character to say "Is there anyone there? Knock once for yes, twice for no!" For a more communicative method of tapping, see Everyone Knows Morse.
Examples:
- There's a radio commercial for Geico insurance which involves "the money you could be saving if you switch to Geico", a stack of money with googly eyes. The announcer asks the money to blink once for yes, twice for no, but then he keeps blinking himself and can't tell if the moneypile has been blinking.
- In Death Vigil, Necromancers who use their powers spawn extra eyes on tattooed regions of their bodies. Naturally, this line comes up when Marlene and James interrogate a Necromancer.
Marlene: No, really, any eye will do!
- A different weasel makes a difference
, as in book canon, has one, two, and three horn blasts to signal different things for the Night's Watch. They also use four horn blasts to signal something: "BREACH! BREACH IN THE WALL!"
- Bumblebee in the Transformers films can only communicate by tuning his radio to different stations.
- In Explorers, a group of kids somehow learn to use their home computer to produce and control a spherical forcefield of arbitrary size and position, eventually using it to build a spaceship. The aliens they meet at the end of the film speak entirely in reproductions of pop culture from Earth's TV broadcasts.
- In Dark Passage (1947), fugitive Vincent Parry has just gotten a face-lift from plastic surgeon Dr. Walter Coley. After the operation, Parry's face is bandaged and the doctor tells him not to move his mouth until he is completely healed:
"Now...I'm going to ask you some questions. If the answer is 'yes', just blink."
- Bit in TRON can only say "yes" or "no" (see also Truth in Television), but can also be more emphatic by repeating itself ("yesyesyesyes" or "nonononono").
- Anchorman: The Legend of Ron Burgundy — "Baxter, is that you? BAXTER! Bark twice if you're in Milwaukee!"
- Hannibal Lecter forces a Bound and Gagged victim to communicate with him this way, by blinking his eyes.
- Black Sunday has Kabakov interogating a man who rented a wooden boat to an American Terrorist, and Kabakov wants him to tell who it was. So he hides on the man's boat in the marina, and when he gets on his boat, Kabakov knocks him down and asks him at gunpoint whether he'll tell him want he wants to know, "Blink for yes, die for no." The man blinks.
- A cruel variation in The Revenant, where Fitzgerald asks Glass, who is paralyzed, if he wanted to be mercy killed. All he had to do is blink. And then he stares at Glass until the latter is forced to blink and then proceeds to try and kill him.
- In The Rookies, after the titular rookies' team leader gets paralyzed from a gunshot wound, she communicates with her team through a monitor screen by blinking.
- In Earth to Echo, after Tuck, Munch, and Alex find part of a crashed machine and discover it's sentient and can beep, they ask it a series of questions about where it comes from and who it trusts. Because of its only ability, Munch suggests naming it "Beep," but the others shut that down.
- To Say Nothing of the Dog has a faked seance with rapping. Hilariously, there are two different sets of people trying to fake it, with very different motivations for what they want the "ghost" to be saying.
- In the Lord Peter Wimsey novel Strong Poison, Miss Climpson fakes a seance, as an excuse to search the house of a dying woman for a will that Lord Peter suspects will contain incriminating evidence.
- One of the stories within the story in Gene Wolfe's Book of the New Sun is told by a man who only ever speaks in political slogans. Another woman translates these into eloquent prose, rendering the story intelligible (and moving).
- In Life, the Universe and Everything, Eddie the computer is gagged and Zaphod tells him to make "mmmm" once for yes and twice for no. Leading to a funny dialogue: "Is it dangerous?" - "Mmmm." - "You didn't just say 'Mmmm' twice?" - "Mmmm mmm." - "Hmmmm."
- During Wraith Squadron, Gammorrean pilot Piggy's translator breaks, and none of the others understand the language, so when asking if he's okay, his squadronmates resort to this.
- Fox in Matthew Reilly's Shane Schofield trilogy had to use this technique twice: by tapping her wrist mike when she couldn't move, and coughing when she was under guard. Apparently "one for no, two for yes" is more dramatic, especially if the hero is going to ask "Are you all right?".
- The Incarnations of Immortality series has Sning, the magic snake ring that answers any question the wearer asks of it by squeezing his (or her) finger: one squeeze for yes, two for no, three to indicate that a yes-or-no answer cannot be given.
- Discworld
- Spoofed in Reaper Man. During a seance, Mrs. Cake's spirit guide One Man Bucket gets a bit confused about which is which. Also, he's perfectly capable of speech, and only has to knock on tables because Mrs. Cake's clients expect that sort of thing in a seance.
- Spoofed again in Interesting Times, where prisoners in the Agatean Empire communicate by knocking on the walls. They can talk to each other, and must do so in order to explain the knocking-code to new arrivals, but they knock on the walls because that's what prisoners do, and because officially prisoners can't talk to each other.
- Noirtier de Villefort in The Count of Monte Cristo is completely paralyzed except for his eyes. He communicates by a system of blinks, including two agreed-upon signals meaning yes and no.
- In I Will Fear No Evil, a doctor uses the alphabet system (with vocal noises by a patient with a mouth full of medical equipment) to communicate with the protagonist immediately post-surgery.
- Dick King-Smith's book Ace is about a pig named Ace who has the unique natural talent of understanding everything humans say, and works out a way of communicating with farmer Ted Tubbs by grunting once for "no" and twice for "yes." He briefly ponders expanding on it by devising specific meanings for three grunts, four grunts and so on, but ultimately decides this will get too complicated for them both.
- In Small Favor, Harry Dresden mocks this idea when his Evil-Detecting Dog seems unsure if something's wrong or not. "You know, Lassie would have given a clear, concise message; one bark for Gruffs, two for Nickelheads."
- This trope is played straight in Summer Knight when Harry is comunicating with an inch-tall wyldfae named Elidee. Blinking faelight is the only effect she can produce that Harry can perceive.
- In Ghost Story, the entity which manifests as Eternal Silence can only speak a few pre-set phrases without the sheer psychic shockwave of its "voice" damaging ghost-Harry. To get around this, Harry suggests that it either nod or keep still depending on if its answer is "yes" or "no".
- Ephraim Kishon's buddy Jossele / Erwinke once invents a code like this, when his boss forbids making private calls during the work: He and his co-worker will let the phone ring X times without taking the call. 43 times means "have you seen the latest Woody Allen flick already?" 46 times means "I did, but it wasn't that special", and so on.
- In A Song of Ice and Fire, Night's Watch scouts use a horn to signal one of three events: one blast for returning brothers, two for wildlings, and three for Others. Whenever the first blast is heard, the men wait with bated breath for the possibility of a second one turning good news to bad.
- In Black Sunday, Kabakov instructs a man whom he has forced a gun into his mouth to respond. "I want to ask you some questions, are you willing to answer? Blink for yes, die for no."
- In The Last Dogs, this is how the dogs communicate with Dr. Lynn, Madame Curie's owner. Being injected with the Praxis serum, they're able to understand her language, so she asks them questions using this trope: barking once for no and twice for yes, for instance. This is also how Dr. Lynn finds out about her dog's death.
- In Void Domain, when Arachne hides as a small spider on Eva's person, tapping her left shoulder is no while right shoulder is yes.
- After she comes out of her coma, My Sweet Audrina tries to blink her eyes to let her father know she's conscious, but she's so weak she can't even control her eyelids.
- In a The Babysitters Club, after Claudia's grandmother Mimi has a stroke, they are relieved to realize that she can still communicate by blinking her eyes until she's able to communicate in other ways.
- In Reborn as a Vending Machine, I Now Wander the Dungeon protagonist Boxxo has been reincarnated as a sapient vending machine, who can only say a few canned phrases. He manages to still make friends with a young adventurer called Lammis after she works out a system with him which she teaches others - for example he uses "Hello there!" to mean 'yes', and "Too bad!" to mean 'no', he also uses "You might win an extra item!" to alert Lammis of approaching enemies.
- In Dr. Franklin's Island Semi is turned into a manta-like creature but retains her human eyes. She can use her soft, toothless mouth and the paddles on either side to manipulate objects a little bit, but it's awkward and slow. When a guilt-ridden scientist decides to help her escape he resorts to asking her to lash her tail once or twice.
- In Rubbernecker, this is used by some of the higher-functioning coma patients on the neurological ward.
- Sam is suffering from pneumonia and severe chest pain. A doctor tells him to blink twice if he's in pain. He blinks many times.
- Meg volunteers to read to Mrs Deal, who never moves except to tap her finger. Meg wonders if she's trying to communicate. She tells her to tap once for yes and twice for no. Mrs Deal taps eight times. Meg concludes that she isn't really conscious and her finger is just twitching randomly. It turns out she's trying to communicate by tapping out letters.
- One episode of Andromeda has Dylan sending a message to the ship, ordering them to arrive at his location. Tyr remarks that, judging from the increased blinking, there is something he's not telling them. Rommie counters by saying he does - the blinks are a High Guard Covert Distress Code.
- Breaking Bad: Hector "Tio" Salamanca is confined to a wheelchair and unable to speak in his old age due to a stroke he suffers during the events of Better Call Saul. He presses a bell attached to his wheelchair arm once for "yes" and does not press it for "no." This unusual method of communication is used for several humorous scenes as he tries to convey his meaning to other characters but is sometimes used more dramatically, as he reveals he knows more than his senile appearance lets on.
- Most dramatically in the season 4 finale, when Walt connects a bomb to the bell. When Gustavo Fring arrives to kill Hector with a lethal injection, Hector activates the bomb by rapidly pressing his bell, instantly killing himself and Tyrus and fatally blowing half of Gus's face off.
- This is used in Community with Abed and a Secret Service agent keeping tabs on him, reversed so two car honks is yes.
- One episode of Criminal Minds featured a victim who was nearly paralyzed. When he tried to write, he only managed to scribble. Morgan has the nurses bring in a communication board so they can get a little more detail than "yes or no." It has the letters of the alphabet in a graph, so Morgan asks him to blink when he reaches the proper line then the proper letter to spell out his responses. He somehow confuses a tired, premature blink for an attempt to spell "Dani" instead of "Dana," despite the A coming before the I.
- CSI: NY: Mac inverts it twice.
- "Blink," the pilot episode, has him using this with a woman suffering from locked-in syndrome. Since she can't move anything but her eyes, Mac asks for two blinks for "yes" and one for "no." Unfortunately, she has a stroke before he can finish questioning her.
- He inverts it again in season 7's "Damned If You Do," only he asks a very badly beaten woman to move her finger instead of blinking.
- Crichton invokes this in an episode of Farscape in order to communicate with a DRD. He even calls it the Star Trek method and names the DRD "Pike".
- In an episode of Full House, Jesse asks Wake Up, San Francisco co-host Rebecca Donaldson on a date. She gives him her answer by winking on the show the next day, with one wink for "yes" and two for "no."
- In one episode of House, the medical team connects a man with locked-in syndrome to a computer controlled by his brainwaves, allowing him to move a cursor to yes, no, etc. And that's after he couldn't blink anymore.
- In an episode of Law & Order: Criminal Intent, a semicomatose patient's therapist gets killed. The detectives suspect the woman's husband, who was trying to disprove the therapist claims about her level of consciousness so that he can withdraw her life-support system and collect her hefty life insurance policy. To get him to confess, they have the dead therapist's assistant set up feedback from the vegetable, using "Yes" and "No" cards and detecting brainwave spikes for a "Yes" if she doesn't look at the signs. Subverted in that it really doesn't work - they're relying solely on the perp's guilty conscience, since the therapist's assistant explicitly states this won't work to the detectives.
- In Law & Order: Special Victims Unit, this also shows up in an episode concerning a multiple sclerosis patient who is reduced to blinking twice for yes and once (or crying) for no. Most of the latter half of the story is largely concentrated on the detectives trying to get her to use her fingers to point at the guilty suspect, as this trope is too weak to use in a court case.
- M*A*S*H: The compound is besieged by artillery fire (and friendly fire at that), during which the officer's latrine gets hit with Henry in it. As the docs and personnel gather at the scene, Klinger invokes this to quantify Henry's status. When Henry knocks twice, Klinger says "Oh God, he's dead!"
- In the Mission: Impossible episode "The Town", a drugged and paralyzed Dan uses this to communicate with Rollin to let him know that his "doctor" is actually holding him prisoner. After the immediate danger is dealt with, they switch to blinking in Morse Code so that Dan can pass on more complex messages.
- Monty Python's Flying Circus. A coffin is brought into court. After the prosecutor asks the dead man inside it a question, a "bang" is heard coming from it.
Judge: What was that knock?Prosecutor: It means "yes" m'lud. One knock for "yes," and two knocks for "no."
- Eventually the prosecutor asks a question; getting no answer, he looks inside the coffin, drops the lid, and says "No further questions, m'lud."
- At the end of Monty Python Live in Aspen (1998), Graham Chapman's ashes use this method of communication to bring Monty Python's The Meaning of Life from beyond the grave. The urn then taps out the melody for "Always Look on the Bright Side of Life."
- One episode of Murdoch Mysteries features a man suffering from aphasia who communicated with ringing a bell. Once for yes, twice for no. He was actually faking this medical condition of his.
- Peacemaker (2022): Peacemaker uses this to communicate with the captured Butterfly queen. Vigilante doesn't seem to grasp that it has to be yes or no questions, although Peacemaker isn't much better, since he can only come up with questions based on the plots of alien invasion movies. They end up with no useful information by the end of it, although it does play into Goff's Hazy-Feel Turn since she was grateful that Peacemaker showed her kindness.
- Person of Interest. Finch is shown communicating with the Machine in its early days via this method, using the vibration setting on his mobile phone.
- Sabrina the Teenage Witch: In "What a Witch Wants", Sabrina tries to invoke this trope while calling Salem to keep Aaron from finding out that he can talk, but Salem instead talks normally because Aaron is answering the door at the moment.
Sabrina: Listen, Salem, if you ever heard of wishes spontaneously coming true, meow once for "Yes", twice for "No".Salem: Not necessary. Lover boy’s busy paying for my pizza.
- Captain Pike, in the Star Trek: The Original Series episode "The Menagerie," is so grievously injured by an accident that he can only communicate by making his specialized wheelchair beep, once for "yes" and twice for "no". At one point he's found repeating "no" over and over again to warn of something he has learned.
- The nebula alien in an episode of Star Trek: Voyager communicates with the crew via the limited phrase book of the ship computer.
- In the Star Trek: The Next Generation episode "Darmok", an alien tries to communicate with Picard using a language comprised solely of metaphors. In practice (and to keep the audience from getting confused), this involves trying to communicate using only a handful of phrases over and over.
- Trapped in another dimension in Stranger Things, Will is able to communicate with his mother by manipulating electrical currents. He initially uses this method with a string of Christmas lights until a slightly more sophisticated means of communication is devised.
- Superior Court: An episode of this 1980s courtroom drama had an episode where a witness, rendered a vegetable after a game of Russian roulette, testify at a murder trial by answering yes/no questions through blinking. His testimony helps convict the defendant of reckless use of a firearm. The post-script is tragic: The young man dies shortly after the events of this episode.
- In the Supernatural episode "I Think I'm Gonna Like It Here" (S09, Ep01), Dean has Crowley locked in the trunk of the Impala. He asks Crowley to knock once for yes and two for no and then asks Crowley if he is alive.
- Jack the dog in Tales of the Gold Monkey: barks once for "no," and twice for "yes." And he's always right.
- In Taskmaster contestants had to figure out what Alex was wearing in the other room.
Alex had a horn to communicate and 4 out of the 5 contestants used once for yes, twice for no while a fifth did it the opposite way.
- In an Unsolved Mysteries segment, a woman is shot in the face. Since she can't speak, the cops and medical staff painstakingly use this to garner information about her assailant—"Did you know the person?" "Was it a man or a woman?", etc.
- The early-90's UK kids' show Woof! has this when the main character becomes a dog.
- An episode of The Odd Couple (1970) has a "seance", where Oscar plays a joke on Felix by doing the knocks (in another room), pretending to be the ghost of the previous tenant.
- "Knock three times on the ceiling if you want me/ Twice on the pipe if the answer is no..." - the classic late 60s hit "Knock Three Times" by Tony Orlando and Dawn, prevents the "yes yes" dilemma by using different objects for yes and no.
- "One blink for yes, two blinks for no / Sweet dreams, sweet cheeks, we leave alone" - Sweet Dreams, Sweet Cheeks by Los Campesinos!!. Implied to be a girl in a coma or paralytic state being asked if she wants euthanasia.
- The Jemjammer party encounter a floating eyeball watching them on Iolara and try to communicate with it by telling it to blink once for yes, twice for no. When Kit informs them that it's just a ball, no lids, they tell it to spin once for yes, twice for no. That ends up working.
- Late 70s BBC Radio sketch series The Burkiss Way used this once. It went something like:
Interviewer: Well with us now we have a distinguished expert in life after death. He’s head of the Practical Séance Department at Cambridge University, Professor Leibniz Ectoplasm. Professor Ectoplasm, in your expert opinion, is life possible after death? (Long pause) ...Er, give one knock for "yes", two knocks for "no".(knock, knock)Interviewer: So there we are: "Yes yes," says Professor Ectoplasm.
- Jasper Carrott does the seance joke version of the trope ("Is there anybody there? Knock once for yes, twice for no!")
- Blithe Spirit has a seance in which the medium's spirit guide knocks once for "yes", twice for "no", and occasionally several times in a row for "I'm in a bad mood and feeling uncooperative".
- New Mombasa's Superintendent Municipal AI in Halo 3: ODST can only communicate via pre-recorded audio city notices and traffic signs. For example, when attempting to ask marines to not blow up a bridge, it asks them to 'Keep It Clean, Respect Public Property'. When faced with the eventuality that the Covenant would soon be accessing its datacenter, the AI releases the bridge controls and ironically responds 'Bridge Toll Accepted, Have a Pleasant Trip.'
- In Knights of the Old Republic, you can suggest for Bastila to help you with the Karma Meter by blinking once for Light Side, twice for Dark Side. She's not amused.
- The Lost Crown: In Nigel's first conversation with Verity, the ghost of a little girl, he asks her to knock twice for yes, once for no.
- In Dark Fall 2: Lights Out, Benjamin Parker must have a similar conversation with Nigel's former partner, Polly White, before she can trust him and show him where her other pair of ghost-hunting glasses are. For some reason this was taken out in the Director's Cut, and the same map, while still available, can be easily missed.
- In the final puzzle of Escape From Ravenhearst, Emma and the Somersets are all Bound and Gagged, unable to signal you except with their eyes. Luckily, they can still give you the code for a mechanism that will free them and win the game: Rose and Emma, by looking up/down or left/right, and the twins by blinking.
- The Witcher 3: Wild Hunt: In the side quest "Fool's Gold," Geralt has to talk to a man who was transformed in to a pig by a curse. Geralt tells him to oink once for yes and twice for no.
- Darths & Droids has an explanation for R2-D2's sounds.
- Sluggy Freelance referenced the Star Trek example by having Torg's idea of helping, when he had accidentally transformed Zoe into a camel, be to build her a machine with 'yes' and 'no' lights.
- The Wotch: Will you or will you not lead us to the Wotch? Nod for "yes", shake your head for "please, cut my leg off!"
- Silent Pete from the Machinima Pre Game Lobby only communicates via music clips.
- Communicating in binary.
- Minilife TV: In "Baking Bran (PART 1)", Michael gets put in a food coma after eating a batch of muffins with an addictive curse known as the Heisen Hex, which leaves him unable to talk. He's given a bell to respond to questions, one ring for "yes" and two for "no".
- Neopets: "Clop" is an unspeaking prisoner who communicates only by knocking his hoof on the floor. One is yes, two is no, three means he's hungry... Too bad nobody knows what four and five mean.
- Pirates SMP: Since Cruppy, a mysterious sea urchin-like creature which the cast find on Day 1, doesn't speak; the pirates usually converse with it by telling it to jump once for yes and twice for no in response to their questions.
- SCP Foundation: SCP-3082
: When 3082-2 is communicating with a somewhat broken drone:
Can you spin this gadget's blades one time for "no" and two times for "yes"?
[The drone is heard activating its motors twice in quick succession.]
- Parodied in Futurama where Zapp Brannigan tells Fry in a Captain Pike-like devicenote to beep once for guilty, twice for not guilty. After Fry beeps twice, Zapp concludes, "Double yes! Guilty!" Leela doesn't hesitate to correct him. Fry spends the rest of the segment communicating with Morse code, and eventually slips back to verbal communication after the Flashback, where Brannigan reminds him to use the beeping noise. Other characters switch out with Fry in the device during their own testimonies, and it's eventually shown that a pedal is used to make the beeping noise.
- In a South Park episode an ex-teacher could only move through a machine which could also beep. (The machine is a reference to the disabled Captain Pike's extremely similar machine from Star Trek.) Everybody thought she was giving everybody the silent treatment when the batteries died. and is asked by the police officer whether a suspect was involved in a crime she witnessed, he interprets her bleeps as "yes, yes!" The commentary specifically says that they just wanted to make the "Yes, yes" joke with her.
- In The Simpsons Bart is diving looking for a treasure and arranging to alert Abe if he needs more oxygen or needs to be raised... by pulling a rope 63 times if he's out of air and 64 if he finds the treasure.
Abe: 61... 62... 63... Oh no! 63! He's out of air! I've sent my only grandson to a watery graaaav... 64! He's found the treasure! I'm rich!
- In another episode ("The Last Temptation of Krust"), Krusty wakes up in Bart's room after a drinking binge, and with his weakened state he tells Bart that they should devise a code of eyeblinks to communicate. Bart immediately starts winking out a response, to which Krusty protests, "Not you!" In the same episode the comics are talking about Krusty's stand-up routine, Janeane Garofalo says "did you get a load of Corpsey the clown?" Bruce Baum says "that guy cheapens our whole profession. What did you think of him, Internet Comic?" then his laptop on a stool makes a sound twice (Apple's Sosumi sound) indicating "no" or "bad" and all the comics laugh.
- In a Treehouse of Horror episode ("The Diving Bell and The Butterball"), Homer is paralyzed completely by a black widow bite and has to listen to Lisa talk too much until it occurs to him he can stop her...by farting. After a little while, Lisa discovers she can translate the number of farts into letters, instead of the usual yes or no. They end up writing a long letter to Marge.
- This happens in Ivor the Engine when his crew try to communicate with him early on. Except there they tell him "One for no, two for yes."
- In Sonic Underground Manic uses this to communicate with Knuckles' pet dinosaur. "One bark means yes, two barks means no."
- The TaleSpin episode "All's Whale That Ends Whale" features a trained whale. When Baloo and Kit discover that the whale's sleazy trainer has been mistreating him, the trainer alters the whale's responses to keep from losing his meal ticket.
- Banana Joe of The Amazing World of Gumball ends one episode injured, to the point that bandages cover his face, because of Darwin and Gumball's negligence. On the bus, they give him a bell to communicate and tell him one rings is yes, two is no. When they asks him if he's still mad, he tries to hit it once but the bus hitting a bump makes him hit it twice. Gumball and Darwin both hug Banana Joe, and he seems resigned to forgive them anyway.
- Truth in Television for sincere attempts at communication in Modern Spiritualism, dating back to the beginning of that faith in 1848.
- Stephen Hawking lost almost all control of his body, and could only communicate with a computer through feeble squeezes.
- Binary Code (computer language) is made up of an extremely complex combination of ones for "Yes" and zeroes for "No", which proves that any concept able to be expressed as a word can be boiled down to those two utterances.
- Note that in one of the simplest applications of this concept, "1" is an active signal, and "0" is no signal. Continuous Wave (CW) is a radio communications technique that relies on this to, among other things, send signals in Morse Code, with the "dits" and "dahs" being 1s of varying lengths.
- Jean-Dominique Bauby (author of The Diving Bell And The Butterfly) suffered a massive stroke and was left with control over one eye. At first he could only answer "yes" or "no" questions but his doctors devised a new system of reading out letters. This meant he was able to write his novel one blink at a time.
- The legendary "one if by land, two if by sea" quote from the American Revolution is one of the most famous historical examples, where a guy is supposed to hold up one lantern if the British are approaching by land, and two if they are coming via river (sea).
- An atypical example: in Japanese, "un" means agree while "uun" means disagree.
- May be used in trying to communicate with paralyzed persons who can't otherwise speak or write, ie those with 'locked in' syndrome, where only the eyes can move.
- American prisoners during The Vietnam War would devise codes they could use to send messages back and forth by knocking on the wall between their cells. Since they couldn't actually tell if the guy in the next cell was a fellow prisoner or a guard trying to trick them, they would sometimes resort to tapping out "Shave And A Haircut" to see if their neighbor would knock twice for "two bits" or not.
- Another example from the Vietnam war, American prisoner Jeremiah Denton was forced by his captors to participate in a televised propaganda interview describing how well he was treated. As the interview went on, he would feign being blinded by the production lights and blink "T-O-R-T-U-R-E" in Morse Code repeatedly. This confirmed for the first time to U.S. Naval Intelligence that American P.O.W.s were in fact being tortured
- Ricardo Woods shot David Chandler in the face and neck in Cincinnati in 2010, leaving him paralyzed from the neck down, on a ventilator, and only able to communicate by blinking his eyes. Before Chandler died, he blinked his eyes three times for "yes" when police showed him a photo of Woods and asked if that was the man who shot him. In June 2013, Woods was sentenced to 36 years to life - a year less than the maximum - for the murder and other charges.
- Commonly used in séances, particularly by fraudulent "mediums" who don't want the hassle of disguising their own voices or relying on pre-recorded dialogue to simulate 'messages from beyond'.
- In addition to eyes, one may also use a system of hand squeezes as a means of communication if they're unable to speak (i.e., for someone in a coma to see if they can hear a person on the "outside" or not.)
- A baseball example: In 2019, it was revealed that during the Houston Astros' World Series-winning season in 2017, they had used such a system as part of an elaborate scheme to use technology to steal their opponents' pitching signs, relayed between the pitcher and catcher. For background, it's legal for teams to try to decode opponents' signs; teams will try to do so when they get a runner to second base, giving him a direct view of the catcher. What's illegal is the use of added technology to do this in real time. The Astros used a video camera in the centerfield seats (also with a direct view of the catcher) to film the catcher's signs and sending the live video feed to a TV monitor directly behind the dugout, where players or team staffers decoded the signs. This trope was invoked when said players/staffers used various audio cues, most notably banging on a trash can with a baseball bat, to let Astros batters know what type of pitch was coming.