Young Jim, his father, and the mattress (pp. 500–501)
It was then that it occurred to me that when I'd been supporting the bed's raised mattress with my chest and face preparatory to its removal from the room, the dihedral triangle I'd imagined the mattress forming with the box spring and my body had not in fact even been a closed figure: the box spring and the floor I had stood on did not constitute a continuous plane.
Context
His father has just collapsed face-down in the dust, and the boy's mind goes to geometry: the figure he'd imagined himself part of never closed. That failed closure is the negative space the doorknob fills two pages later — the perfectly self-closing figure arriving right after the family one falls open.