Viewers of The Big Bang Theory are familiar with Dr. Sheldon Cooper's famous triple knock. What is the origin and reason for this humorous (and slightly irritating - from the characters perspective) behavior?

  • 3
    Are you looking for an in- or out-of-universe explanation?
    – JAD
    Commented Feb 1, 2018 at 13:24
  • Any! I just wanted to know if there was a reason behind it, or if it is just an ongoing joke throughout the series. Commented Feb 1, 2018 at 13:52
  • Well, it's a comedy serie, so a lot of things are very easily explained as being a joke ;)
    – JAD
    Commented Feb 1, 2018 at 14:16
  • The triple-knock itself, or, why he does it three times? With Penny, he does it three times in rapid succession, because she refuses to answer until after the third time, because then it meant that it was about something important. It had to do with him pestering her so much over trivial things. Commented Feb 2, 2018 at 14:15

2 Answers 2


From wikia:

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 his father 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.

Because Wikia is, well, Wikia, here's a link to a more reliable transcript of the episode: link

  • 4
    Transcript Commented Feb 1, 2018 at 17:23
  • Assuming Young Sheldon lasts long enough (which seems likely, it's a big hit), I wonder if they'll recreate that incident.
    – Barmar
    Commented Feb 2, 2018 at 1:27
  • 3
    Also, obsessive compulsivity would explain why he often continues to do it even though it may be unnecessary in a certain situation. Commented Feb 2, 2018 at 4:54
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    @can-ned_food can't recall which season/episode, but Sheldon has closure issues and at one point Penny opens the door before Sheldon could knock-knock-Penny the 3rd knock, ...he closes the door and resumes with the 3rd knock-knock-Penny. There's definitely an OCD involved here. Commented Feb 2, 2018 at 18:16
  • @Mat'sMug This behavior is a caricature of that of someone with an Autism Spectrum Disorder (e.g. Asperger's syndrome), not OCD; Despite having some superficial similarities, there are very fundamental differences between the two. Commented Feb 3, 2018 at 11:41

While Plexus's answer is absolutely correct, it should be noted that there is a bit of an inconsistency in the show. Sheldon claims that he started using the triple knock after this traumatizing event, but it can be seen in the first few episodes of season 1 that he only uses a normal "one-time knock" on Penny's apartment door. The triple knock first appears in the show a few episodes into season 1.

  • 10
    That's just what TV Tropes calls "Early Instalment Weirdness"; it's quite common in sitcoms. Basically, they hadn't come up with that particular quirk when they started writing the show, and invented it later.
    – F1Krazy
    Commented Feb 2, 2018 at 11:43
  • 6
    That might not actually be out of character as a retcon... In the beginning, he may not have formed his perception of Penny's promiscuity, but once he did, he started using it at that point... Then all the heathens around him started having naked fun time, so he just started doing it all the time :)
    – Taegost
    Commented Feb 2, 2018 at 13:41
  • If I remember correctly, in about the third or fourth episode of season 1 Sheldon does the knock-"Penny" repeatedly (about 6 or 7 times). This got a big laugh, so I assume they started repeating it. I'll try & find the exact episode later.
    – Dragonel
    Commented Feb 2, 2018 at 17:28

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