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Memory Alpha

Knocking was the practice of announcing one's presence at a door (and therefore, one's desire to enter same and/or speak with the occupant behind it) by striking its surface with one's fist. Not to knock first was often considered rude.

In 2004, Jonathan Archer asked Loomis if he made it a habit of climbing out the window of his apartment in Detroit every time someone knocked on his door. Loomis replied that he did not like the police. (ENT: "Carpenter Street")

In 2154, Captain Archer planned to fly a Xindi-Insectoid assault shuttle on a suicide mission to Azati Prime to destroy the Xindi weapon. T'Pol asked Archer to reconsider his decision and he sarcastically asked her what his options were, fly a shuttle into the system, knock on the door, and ask the Xindi if they wanted to talk. (ENT: "Azati Prime")

In 2266, Janice Rand admonished Charles Evans not to walk into a room (particularly her quarters) without knocking. (TOS: "Charlie X")

In 2293, Captain James T. Kirk noticed Lieutenant Valeris standing in the doorway to his quarters on the USS Enterprise-A while he was dictating his captain's log. She apologized and Kirk responded that she could have simply knocked on his door instead. (Star Trek VI: The Undiscovered Country)

In 2365, Commander William T. Riker provided Acting ensign Wesley Crusher with advice on how to properly speak to a woman, while talking with Guinan in Ten Forward. During their conversation, Guinan told Riker that there was nothing wrong with a line, saying that it was like a knock at the door. Later that year, Riker recalled this conversation when he was infected by Surata IV microbes. (TNG: "The Dauphin", "Shades of Gray")

In 2366, Commander Riker, upon determining that a frontal assault was unwarranted, decided to try an old-fashioned knock on the front door of the Uxbridge house after beaming down with an away team to Rana IV. (TNG: "The Survivors")

In 2370, K'mtar asked Commander Riker if Quark could have been lying to him about the location of the Duras sisters. Riker told him that he would just be knocking on Quark's door again in a few days if he were and he would not be in as good a mood. (TNG: "Firstborn")

In 2372, while temporarily stranded on Planet Hell in the Delta Quadrant, Tom Paris told Neelix that if he were not the one dating Kes that he would be the first one knocking on her door. He told him he would not do that as he respected the Talaxian. (VOY: "Parturition")

In 2375, Ezri Dax told Elim Garak that if he simply wanted to get some fresh air, he for sure was knocking on the wrong door after he attempted to open an airlock into space whilst suffering from severe claustrophobia. (DS9: "Afterimage")