Sheldon's spot is the position on the couch he has claimed as his own, putting it in a "state of eternal dibs". Sheldon has described his spot as the "single point of consistency in an ever changing world", and has even said that his feelings for his spot are much greater than those for his mother.
In the Pilot episode, Sheldon explained to Penny that he chose his spot as "in the winter that seat is close enough to the radiator to remain warm, and yet not so close as to cause perspiration. In the summer it's directly in the path of a cross breeze created by open windows there, and there. It faces the television at an angle that is neither direct, thus discouraging conversation, nor so far wide to create a parallax distortion."
In The Tangerine Factor, Sheldon chose a place to sit in Penny's apartment for the first time. After examining the options, he settled on the chair next to her couch. Years later, in The Infestation Hypothesis, when Penny took in a new chair, Sheldon was initially pleased with the chair, until he found out the chair was taken off the street. Despite Sheldon's best efforts, Penny refused to get rid of the chair, until a rat inside bit Amy on the keister.
Before Leonard moved in an bought a couch, Sheldon had a lawn chair in the same position. In The Staircase Implementation, Sheldon explained that it "faces the television at a direct angle allowing me to immerse myself in entertainment or game play without being subject to conversation". Sheldon explained he had placed the chair in a state of eternal dibs and, when pressed on it, said "Cathedra mea, regulae meae. That's Latin for 'my chair, my rules'."
When Penny accidentally shot Sheldon's couch cushion with a paintball gun in The Cushion Saturation, Sheldon said that if his life were expressed as a function in a four-dimensional Cartesian coordinate system, his spot, when he first sat on it, would be "0,0,0,0".
In The Apology Insufficiency, Sheldon offered Howard his spot on the couch as an apology for costing him a government job. In the end, however, Sheldon only let Howard sit in his spot for 94 seconds before demanding it back.
When the guys were on an expedition in the Arctic in The Monopolar Expedition, Sheldon claimed the same spot on a couch in their cabin. After being sent to jail in The Excelsior Acquisition, Sheldon claimed the left side of the bench. In The Rothman Disintegration, Sheldon told Kripke he was in his spot at the urinals.
In The Parking Spot Escalation, Sheldon and Howard had a bitter fight after the university reassigned Sheldon's parking spot to Howard. During a series of tit-for-tat reprisals, Howard performed a "nude revenge wiggle" on Sheldon's couch cushion.
In The Fortification Implementation, after Sheldon and Amy erected a fort in the living room, Sheldon sat down at Amy's suggestion. When Leonard returned to the apartment, Sheldon invited Leonard into the fort to take a seat on the floor. When Leonard sat down, Sheldon told him "Not there. That's my spot."
When Sheldon feared he was losing his mind in The Earworm Reverberation, he recorded a video for "Crazy Future Sheldon" explaining the most important points of his life, including the ideal thermostat setting and his spot on the couch. The only other person Sheldon was willingly let sit in his spot is his Meemaw (The Meemaw Materialization).
Sheldon's protectiveness over his spot doesn't just extend to people sitting there. In The Application Deterioration, Sheldon tells Amy to scoot over as part of her shadow was on his spot. In The Fermentation Bifurcation, after an enjoyable evening of Dungeons and Dragons with a pregnant Bernadette, Sheldon invites Bernadette over to the apartment whenever she wants a break from being "Bernadette the Pregnant". Sheldon stipulates that the offer is only good until the third trimester as he can't risk getting amniotic fluid on his spot.
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Sheldon tells Penny why he chose his spot on the couch: "In the winter that seat is close enough to the radiator to remain warm, and yet not so close as to cause perspiration. In the summer it's directly in the path of a cross breeze created by open windows there and there. It faces the television at an angle that is neither direct, thus discouraging conversation, nor so far wide as to create a parallax distortion."
When Penny wanted to talk to Sheldon about her upcoming date with Leonard, she invited him into her apartment and asked him if he wanted to sit down. Sheldon said it wasn't that easy as he hadn't spent much time there and wasn't "familiar enough with the cushion densities, air flow patterns and dispersion of sunlight to make an informed choice" about where to sit. After examining the options, Sheldon eventually settled on the chair by Penny's couch.
Sheldon describes his spot on the couch as a single point of consistency in an ever-changing world. He says if his life were expressed as a function on a four-dimensional Cartesian coordinate system, that spot at the moment he first sat on it would be 0,0,0,0.
At the cabin in the Arctic, Sheldon chooses the same spot on the couch as he does in his apartment.
In the prison cell, Sheldon chooses the same spot on the bench as he does on the couch in the apartment.
When past Sheldon and Leonard sit down to iron out the Roommate Agreement, Sheldon explains why he placed one of the lawn chairs in the living room in a state of "eternal dibs". The reasons are similar to those he would later state for his spot on the couch, although a key difference is that past Sheldon wanted a seat facing the television at an angle that discouraged conversation.
Later in the episode, Leonard buys a couch for the apartment. Sheldon is initially upset at the purchase but he still sits on the couch and proclaims "this is definitely going to be my spot". Though, as Penny pointed out, there was no way for Leonard to know Sheldon said that as he had left the room.
When Sheldon failed to get Howard to accept his apology for costing him a government job, Sheldon offers Howard his spot on the couch as a peace offering. Howard accepted Sheldon's apology, but in the end Sheldon only let Howard sit in his spot for 94 seconds before demanding it back.
When Penny took in a new chair, Sheldon was initially pleased with the seat. After Penny admitted it came from the street, germaphobe Sheldon begged Penny to remove the chair from the building. Despite Sheldon's best efforts, Penny refused to get rid of the chair, until a rat inside the chair bit Amy in the keister.
During Sheldon and Howard's feud, Howard exacts revenge on Sheldon by sitting in his spot naked with Sheldon's laptop resting on his groin. Sheldon later describes the incident as a "nude revenge wiggle".
When Sheldon invites Leonard in to his and Amy's fort in the living room, Amy tells Leonard to have a seat on the floor. Sheldon immediately tells Leonard "Not there. That's my spot."
When Sheldon films a video for "Crazy Future Sheldon" complete with information about his life, Sheldon points out his spot and explains "You're very protective of it. When anyone else tries to sit here, you berate them relentlessly. It sounds mean, but somehow you make it adorable."
When Amy arrives at the apartment to meet Sheldon's Meemaw, Amy finds her sitting in Sheldon's spot.
"Amy: And look at you, sitting in Sheldon's spot. You know, I don't even get to sit there.
Sheldon: Yeah. And you never will."
When Penny suggests Sheldon add a clause to his contract preventing him from making fun of Howard, Bernadette wonders how such a clause would be enforced. Sheldon says it would be enforced by his own personal code of ethics, but Amy points out there's also his obsessive-compulsive disorder. Sheldon agrees with Amy and then asks her to scoot over as part of her shadow's on his spot.
After spending an enjoyable evening playing Dungeons and Dragons with Bernadette to get her mind off being pregnant, Sheldon tells Bernadette that any time she wants a break from being "Bernadette the Pregnant", "Bernatrix the Warrior Queen" is waiting. When Bernadette says she might take him up on that offer, Sheldon points out that it's only good until the third trimester as he can't risk getting amniotic fluid on his spot.