When Leonard bought a couch, the left hand side of the couch (when sitting on it) became Sheldon's spot.
While initially Sheldon did not want to be able to engage in conversations when sitting in his spot , three years later Sheldon's explanation to Penny why he chose this particular spot had slightly changed. He now pointed out that "it faces the television at an angle that is neither direct so that it would discourages conversations nor at an angle that would causes a parallax distortion".
Sheldon describes his spot as a "single point of consistency in an ever changing world". He says that if his life was expressed as a function in a four-dimensional Cartesian coordinate system, this spot, when he first sat on it, would be "0,0,0,0".