Sheldon's Knock

Knock penny — Sheldon using his signature knock on Penny's door

Sheldon has a particular door knock ritual when alerting someone to his presence outside their door. Due to his obsessive compulsive tendencies, he must finish his knock, even if the person he's alerting opens the door before he is finished, in which case he will usually finish it on the door frame.

His knock consists of three knocks before stating the person's name, and then he repeats this process two additional times:

(knock, knock, knock) "Penny?"

This knock is mostly associated with his trips to Penny's apartment across the hallway, but is his general knock for any character in the series. Sheldon will also use this knock on the wall above someone's head if he enters their bedroom and is going to wake them up.

Origins

In the episode "The Hot Tub Contamination", the origins of this knocking ritual are revealed. When Sheldon was 13 years old, he once came home because they ran out of math to teach him. His mother was at a Bible study. He walked in the house expecting it to be empty and heard a sound from his parents' bedroom. When he opened the door, he saw hisfather having relations with another woman. It’s why he never opens a door without knocking three times. The first one is traditional, but two and three are for people to get their pants on.

Series Development

The first time we see Sheldon knock on someone's door was in the second episode of the first season called "The Big Bran Hypothesis", and due to the infancy that the series was in, Sheldon Lee Cooper's knock was nothing unusual. It wasn't until episode ten ("The Loobenfeld Decay") that his knock had become unique. Sheldon's signature knock has gone through several changes in the series before his signature knock was created.

The earliest version of his signature knock was shown in "The Loobenfeld Decay". This featured three or four knocks before saying the person's name, and it continued until they answered. In this episode, it was on Leonard's and Penny's doors. His current knock (three knocks and the name; three repetitions) officially caught on in "The Euclid Alternative" and became his signature knock with few exceptions in which he varied it throughout the series.

In-universe, Sheldon's earliest time to have used this knock was in the flashback episode "The Staircase Implementation", which takes place before the start of the series. It's unclear if his knock was actually different at this time, because the only scene it's featured in depicts it slightly different, being slower. This difference could have been due to the situation of in which he used it.

In "The Habitation Configuration", Sheldon is drunk and knocks on Wil Wheaton's door two times instead of three, then later corrects himself by knocking again in the middle of his conversation.

Sheldon Lee Cooper's knock used by others

Throughout the series, Sheldon Lee Cooper's knock has been mimicked by other characters as either a parody or to make the person behind the door think it is Sheldon doing the knocking:

"The Panty Piñata Polarization" (S2E7): Penny angrily mimics the knock when she confronts Sheldon over her missing laundry.
"The Vartabedian Conundrum" (S2E10): When Sheldon thinks he lost his voice, he uses a computer-generated voice in place of his own to say "Penny".
"The Hot Troll Deviation" (S4E4): Howard uses a recording of Sheldon saying Penny's name to copy his knock so Penny would think it was Sheldon, because he knew she wouldn't answer the door if she knew it was him and wanted an update onBernadette.
"The Isolation Permutation" (S5E8): Leonard knocks on Amy's door like Sheldon in order to make Sheldon stay and talk to her.
"The Monopolar Expedition" (S2E23): Sheldon is chosen to go to the magnetic North Pole to try and confirm string theory. Leonard, Howard and Raj are allowed to join him. Sheldon uses this knock twice for Penny. The second time, she stands on the other side of the door and repeats his knock to him, saying his name. She finally opens the door after several times.

Penny!
"The Cooper Extraction" (S7E11): While Sheldon is away for the holidays, the guys throw a Christmas party and talk about the movie "It's a Wonderful Life," and Penny asks if Leonard could imagine what that would be like to never have been born, to which Leonard responds by imitating Sheldon's knock, only instead of a name, saying "What do you think?" each time.
"The Communication Deterioration" (S8E21): Penny uses Sheldon's knock as a joke. He concludes that by the third knock, she liked it and she agrees and wants to do it again. Sheldon warns her that if she does, she'll never stop.

Episodes

"The Loobenfeld Decay" (S1E10): Penny is singing in a production of "Rent", and Leonard and Sheldon don't want to have to hear Penny's singing. Sheldon develops an elaborate ruse into why him and Leonard can't go to Penny's performance. Sheldon uses the earlier version of his knock four times in the episode: three times with Leonard and once with Penny.
"The Euclid Alternative" (S2E5): Leonard works the night shift, so Sheldon can't drive himself to work. Sheldon uses his knock once with Penny. This is the first time that he uses the knock that he is known for later on.
"The Cooper-Nowitzki Theorem" (S2E6): Sheldon has a woman at the University who is interested in him and in working with him. Sheldon uses this knock once with Penny, and she makes mention that he doesn't need to knock so much as she can hear him due to her small apartment.
"The Panty Piñata Polarization" (S2E7): Penny is given a third strike and thus banished from the apartment by Sheldon; Penny and Sheldon fight. Penny uses it once to ask him where he put her clothes that were in the dryers in the laundry room. Sheldon uses it once for Penny to concede (Leonard had gotten Penny to call Sheldon's mother and tattle on Sheldon to end the fight).
"The White Asparagus Triangulation" (S2E9): Sheldon tries to set up Leonard with Dr. Stephanie Barnett. He uses the knock three times; once with Leonard (to show Leonard that Stephanie Barnett's relationship status on her web profile was still 'single') and twice with Penny (to get advice from Penny on how to get Leonard and Stephanie together).
"The Vartabedian Conundrum" (S2E11): Dr. Stephanie Barnett performs the "Sheldonectomy" - tricks Sheldon into thinking that his larynx is inflamed and must stop talking immediately. Sheldon uses his knock once with Penny, but uses a voice program on his laptop to say her name.
"The Friendship Algorithm" (S2E13): Sheldon tries to scientifically discover the secret of friendship, so that he can become friends with Kripke. He uses his knock once with Penny, handing her a multi-page friendship questionnaire.
"The Work Song Nanocluster" (S2E18): Penny tries to become self-employed, making Penny Blossoms with Sheldon's business knowledge assistance. Sheldon uses this knock once for Penny. He is delivering a package that was left with him when she wasn't home.
"The Monopolar Expedition" (S2E23): Sheldon is chosen to go to the magnetic North Pole to try and confirm string theory. Leonard, Howard and Raj are allowed to join him. Sheldon uses this knock twice for Penny. The second time, she stands on the other side of the door and repeats his knock to him, saying his name. She finally opens the door after several times.
"The Electric Can Opener Fluctuation" (S3E1): Sheldon's work trip to the North Pole was sabotaged by Leonard, Raj, and Howard since Sheldon was becoming unbearable and the only way that to make Sheldon bearable was by producing false scientific results for Sheldon; Leonard and Penny become a couple upon Leonard's return from the North Pole. Sheldon uses this knock once for Leonard (while Leonard and Penny are kissing). When Leonard tells Penny to not make a sound, Sheldon then uses the knock once for Penny.
"The Pirate Solution" (S3E4): The university has cut the funding to Raj's department, and he is going to be deported if he loses his job. He gets a job working for Sheldon. Sheldon uses this knock for Raj.
"The Cornhusker Vortex" (S3E6): Penny has her friends over to watch a football game, and Leonard goes to watch the football game in an attempt to fit in with Penny's friends. Sheldon uses this knock once during Penny's party, even though her door is open.
"The Guitarist Amplification" (S3E7): Penny and Leonard's arguing upsets Sheldon. Sheldon uses this knock once for Raj. Raj yells at Sheldon to come in since the door is open, and then requests that Sheldon tell Raj's parents that their research is at a critical juncture and that Raj can't go home to India to attend his cousin Sanjay's wedding.
"The Adhesive Duck Deficiency" (S3E8): Penny slips in the shower and dislocates her shoulder; Sheldon has to drive Penny to the hospital for medical care. He uses this knock on all the closed doors in Penny's apartment on his way back to her bathroom. She tells him to not knock when he gets to the bathroom door.
"The Maternal Congruence" (S3E11): Leonard's mother comes for a visit and divorces from Leonard's father. Sheldon uses this knock once for Leonard.
"The Bozeman Reaction" (S3E13): Sheldon and Leonard's apartment is burglarized. Sheldon uses this knock four times, all for Leonard and Penny who are together.
"The Einstein Approximation" (S3E14): Sheldon spends days trying to figure out how electrons move through grapheme as if they have no mass. He uses this knock once after entering Penny's bedroom to wake Leonard and Penny up.
"The Excelsior Acquisition" (S3E16): Sheldon gets a court summons from the time he drove Penny to the hospital, Sheldon is determined to fight the charges and Penny will accompany him to court. He uses this knock once for Penny - however before Sheldon can do the third triple knock, Penny opens her door, does the triple knock and says "Penny?", to which Sheldon replies, "Well, that's just wrong."
"The Wheaton Recurrence" (S3E19): A bowling bet between Sheldon and Stuart. Stuart has Wil Wheaton on his team, and Wil Wheaton breaks Penny and Leonard up in order to make Sheldon's bowling team lose. Sheldon uses his knock once to give Penny ice cream (which according to studying the comic strip "Cathy", is appropriate to give to a woman under stress).
"The Spaghetti Catalyst" (S3E20): Leonard and Penny are broken up, Sheldon is divided into whether he should spend time with Leonard and the guys or whether he should spend time with Penny. Sheldon uses this knock once for Penny (Sheldon has only one hot dog left, having had to give up the other five to a mean angry dog in order to save his life). He uses this knock again on the heartbroken Leonard in his bedroom, who tells him to just come in.
"The Staircase Implementation" (S3E22): Flashback episode. Leonard and Sheldon are arguing over the thermostat, and Sheldon goes to Penny's apartment and recounts to Penny how he and Sheldon became roommates. Sheldon uses this knock once for Leonard and Joyce Kim.
"The Robotic Manipulation" (S4E1): Sheldon thinks about having offspring with Amy Farrah Fowler. Sheldon uses this knock once for Penny, after whom she explains that she stands on the other side of the door, waiting for him to finish, implying that it's a waste of time. Sheldon states that he was aware of that, being able to see the shadow of her feet under the door.
"The Cruciferous Vegetable Amplification" (S4E2): Sheldon is worried about getting hurt and wants to exist as a robot. He uses this knock three times, once for Leonard (when he has pain in his abdomen from eating so many vegetables) and twice for Penny (once to go jogging with Penny, and the second time, using his robot in order to ask Penny to sing "Soft Kitty" to him).
"The Hot Troll Deviation" (S4E4): Using his avatar in his World of Warcraft game, Howard cheats on Bernadette. Howard them uses a recording of Sheldon's voice and his signature knock to get Penny to come to the door.
"The Irish Pub Formulation" (S4E6): Leonard and Priya are in a relationship, trying to keep the relationship secret. Sheldon uses his knock once for Leonard (while Priya is in Leonard's bedroom, Leonard tries to keep Priya's presence concealed from Sheldon).
"The Justice League Recombination" (S4E11): The guys befriend Penny's boyfriend Zack and get him to dress up as Superman for the comic book store New Year's Eve costume contest. The guys want Penny to come to the costume contest dressed as Wonder Woman. Sheldon uses his knock for Penny as the Flash, which consists of him knocking and saying "Penny" repeatedly. Before this, Zack gets upset at the guys and storms to Penny's apartment. When the group goes to apologize, Leonard knocks on the door three times, there is a pause, then Sheldon mutters "Penny", to which Leonard replies "Oh, come on!"
"The Love Car Displacement" (S4E13): Sheldon, Leonard, Howard, Raj, Penny, Bernadette, and Amy make a weekend trip to a spa/four-star hotel because of a science conference that is going on. Sheldon uses this once on Raj, after Penny enters his and Leonard's hotel room and he feels no need to stay in there if they will be having "coitus".
"The Thespian Catalyst" (S4E14): Sheldon teaches a class horribly. Amy suggests that teaching is a performance art (requiring the ability to communicate, entertain, and engage), and that Sheldon could be a better teacher if he took acting lessons. Sheldon uses his knock twice, both for Penny (Once to ask Penny to give him acting lessons. The second time he knocks while dramatically saying "Penny?", when Penny opens the door to ask what is wrong, Sheldon replies, "Nothing. I was acting").
"The Toast Derivation" (S4E17): Leonard, Howard, and Raj spend all their leisure time at the apartment where Raj and Priya are, instead of spending time at Sheldon and Leonard's apartment. Sheldon "teaches" Leonard a lesson by hosting a social gathering at Sheldon and Leonard's place, inviting Kripke, Zack, Stuart, and LeVar Burton. Sheldon uses his knock once for all his friends at Priya and Raj's apartment.
"The Prestidigitation Approximation" (S4E18): Howard shows Sheldon a card trick, and Sheldon cannot figure out how the magic trick was done. Sheldon uses his knock once for Leonard, Sheldon wanted to practice figuring out how the card trick works while Leonard and Priya are in bed.
"The Agreement Dissection" (S4E21): Priya finds legal loopholes in Sheldon and Leonard's Roommate Agreement. Sheldon uses his knock twice for Penny (Once so that he can use Penny's bathroom. The second time he knocks to tell Penny that the mean Indian lady tried to make Sheldon eat lamb).
"The Infestation Hypothesis" (S5E2): Penny gets an abandoned chair for her apartment and Sheldon believes its unsanitary. Sheldon uses his knock a total of six times in this episode, once for in a nightmare with Penny, four times in real life with Penny, and once with Amy. His last knock with Amy, she states that his obsessive knocking is indicative of obsessive-compulsive disorder, which Sheldon outright denies.
"The Isolation Permutation" (S5E8): Bernadette and Penny go wedding dress shopping without Amy, upsetting Amy. Leonard uses this knock on Amy to force Sheldon to talk to her.
"The Ornithophobia Diffusion" (S5E9): A bird won't leave Sheldon and Leonard's window sill. Sheldon uses this knock on the window to make the bird go away.
"The Flaming Spittoon Acquisition" (S5E10): Stuart asks Amy out on a date. Sheldon attempts to make Amy jealous by asking Penny out on a date. Sheldon uses this knock three times. He knocks once for Penny, once for Amy to end her date with Stuart, and once for Penny, Bernadette and Amy (while the three are playing Twister).
"The Vacation Solution" (S5E16): University President Siebert forces Sheldon to take vacation. Sheldon prefers to spend his vacation time in Amy's lab and he uses his knock once in there.
"The Weekend Vortex" (S5E19): Amy is going to a family party, but Sheldon doesn't want to go. Sheldon uses his knock for Penny to give her gummy bears.
"The Launch Acceleration" (S5E23): NASA calls Howard to inform him the mission has been cancelled. Howard prepares for his upcoming wedding. Amy wants to take her relationship with Sheldon to a higher level. NASA calls Howard to let him know that the mission will proceed as planned after all. Sheldon uses his knock once for Amy when he arrives for date night.
"The Decoupling Fluctuation" (S6E2): Sheldon is worried that Penny might be considering breaking up with Leonard. He uses his knock once on Leonard in an attempt to tell him about it, but Amy told him not to so Sheldon doesn't. He immediately goes to Penny where he does the first and second part of his knock on Penny's bedroom wall. Penny grunts but doesn't wake up. After the third part, Sheldon says Penny's name much louder, almost in normal speech, and Penny wakes up screaming. This frightens Sheldon, causing him to scream too.
"The Extract Obliteration" (S6E6): Sheldon and Stephen Hawking are playing a tablet game. When Sheldon is winning, Hawking stops playing. After failing to make him return with a Jedi mind trick, Sheldon uses this knock on Hawking, knocking on his tablet.
"The Habitation Configuration" (S6E7): Amy has an argument with Wil Wheaton and gets mad at Sheldon for not choosing her side. Sheldon goes to her apartment and uses this knock on her, but he finishes his final knock with "Angry Amy" instead of just "Amy". Later, drunk of alcoholic tea, Sheldon goes to Will's house to "beat an apology out of him". He uses this knock on Will, only to lose count after the second set of knocks. Mid-conversation, he finally finishes the knock.
"The Egg Salad Equivalency" (S6E12): Alex has accused Sheldon of "inappropriate behaviour in the workplace". He uses this knock on the Human Resources Department. The worker, who is unfamiliar with Sheldon's ritual, just says "Come in?" in confusion.
"The Spoiler Alert Segmentation" (S6E15): Leonard has a fight with Sheldon and moves to live with Penny. Sheldon, looking for a new roommate, is stunned when Amy suggests to become his roommate. Sheldon heads to Apartment 4B and uses this knock, saying "Good buddy Leonard".
"The Tenure Turbulence" (S6E20): Sheldon uses this knock on Mrs. Davis, trying to get tenured. At the end of the episode he wants to express his gratitude to Janine, so he uses this knock again on "Mrs. Davis". She doesn't initially respond, but Sheldon saw her car in the parking lot, thus knowing she was in there.
"The Love Spell Potential" (S6E23): Sheldon and Amy are playing Dungeons and Dragons when Howard gives an awful description about them. Amy leaves to Sheldon's room, who uses this knock on her. He said he never had to knock on his own door before.
"The Hofstadter Insufficiency" (S7E1): After Sheldon wakes up from having a nightmare with Leonard being dragged into the ocean by a kraken (brought on by watching Clash of the Titans before bed), he immediately runs over to Penny's apartment and rapidly does the knock on her door.
"The Deception Verification" (S7E2): Leonard has returned home early from a four-month sea expedition to spend a few days with Penny. Sheldon and Amy barge into Penny's apartment and catch them in the act. When Penny says they can't just barge in, Sheldon closes the door again and uses this knock on Penny.
"The Workplace Proximity" (S7E5): Sheldon uses this knock for Penny's Posse, all of whom are in Apartment 4B discussing Amy's upcoming work at Caltech ("Amy, Bernadette, Penny"). Coincidentally, Sheldon says the girls' names in the order from which they are sitting by the door (closest to farthest). When Bernadette comments on how Sheldon may never stop doing that, Amy replies that she'll use that type of repetition to her advantage someday. Later, he uses the knock at Amy's Apartment when trying to apologize to her.
"The Proton Displacement" (S7E7): Sheldon can't stand Professor Proton chose Leonard for help over him. He goes to his house and uses this knock on him, saying "Arthur".
"The Hesitation Ramification" (S7E12): Sheldon tries to learn how to be funny and starts experimenting with knock-knock jokes. Amy says to Bernadette and Howard that's exhausting. "Do you have any idea how hard it is to laugh at a knock-knock joke that starts with knock-knock-knock, Amy, knock-knock-knock, Amy, knock-knock-knock, Amy?"
"The Friendship Turbulence" (S7E17): Sheldon and Howard are on an airplane flying to Houston, TX. Sheldon needs Howard to move so he can use the toilet. To get Howard's attention, he taps on his shoulder in the knock rhythm, going tap-tap-tap-Howard tap-tap-tap-Howard tap-tap-tap-Howard.
"The Mommy Observation" (S7E18): Sheldon uses this knock on his mother when paying her a surprise visit. He just says "Mother" in the knock.
"The Relationship Diremption" (S7E20): Sheldon gets very drunk and Amy brings him to his room. Before entering, he uses the knock on his room, saying "Empty room". He does that because "If somebody says come in, I’m gonna freak out".
"The Anything Can Happen Recurrence" (S7E21): Sheldon, Leonard & Penny start another "Anything-can-happen-Thursday". They go out looking for a random restaurant, only to discover Amy & Bernadette in one. They had been lying about their whereabouts. Sheldon uses this knock on Amy & Bernadette, tapping the window.
"The Anxiety Optimization" (S8E13): Sheldon does the knock on Penny's door when he hears her grunting in her apartment; he opens the door when she doesn't answer and he realizes that she didn't lock it. Later, while the girls are in Penny's Apartment talking about her job, Sheldon does the knock, saying "Ladies"; Penny simply tells him to come in.

Soft44 — Penny doing Sheldon's knock and then warned that it's habit-forming.

"The Communication Deterioration" (S8E21): Penny uses Sheldon's knock as a joke. He concludes that by the third knock, she liked it and she agrees and wants to do it again. Sheldon warns her that if she does, she'll never stop.
"The Matrimonial Momentum" (S9E01): Sheldon is unable to knock due to having his hands full, so he says, "Knock knock knock Penny, knock knock knock Penny, knock knock knock Penny."
"The Opening Night Excitation" (S9E11): Sheldon uses his knock for Penny. Bernadette says "what if I say come in?". Sheldon does a one second momentary silence and then he knocks for Bernadette's name. Penny then says "Come in!". Sheldon declares he has nowhere else to be. Bernadette says "just come in". Later on, at Amy's apartment he knocks on her door three times as "Birthday girl" and he walks into her apartment, after she unlocks it, of course.
"The Meemaw Materialization" (S9E14): Sheldon uses the knock on Leonard and Penny (who are in her apartment), after he excuses himself from Meemaw's confrontation with Amy.
"The Viewing Party Combustion" (S9E21): Sheldon uses the knock for Leonard on his bedroom door, while he and Penny are trying to sleep, so that he could ask his Batman-Manbat question (which he couldn't do earlier).
"The Line Substitution Solution" (S9E23): Sheldon used the knock for Amy in Apartment 4B to apologize for standing her up when they were supposed to go shopping, and for making Stuart do it for him. He does this while rushing so he can get back in line for the Avengers movie screening.
"The Geology Elevation" (S10E09): Sheldon used the knock on Bert's office door to confront him about his recent Grant accomplishment.

Trivia

So far, Stuart Bloom, Emily Sweeney, and Leslie Winkle are the only characters in the main cast (current and former) that Sheldon has not done his signature knock on.