In "Pit and the Pendulum," the pit symbolizes the unknown horrors that the Spanish Inquisition created. The pendulum symbolizes the inexorable march of time and the imminence of death, the second of which is a common theme throughout many of the works of Edgar Allan Poe.
The narrator of the story is a prisoner of the Spanish Inquisition, and he has been placed in one of the creative torture chambers that this horrific institution devised. The rumors about the methods the Inquisition uses are just as frightening to the narrator as the methods themselves. The darkness in the room keeps the narrator from knowing what the room is, or whether the pit (after he almost falls into it) even has a bottom. This room appears to lack order or structure of any kind, which may be the most frightening torture of all.
The pendulum swings back and forth, bearing down on the narrator's heart. Each swing brings it closer to the narrator, and the rhythm of the pendulum takes on the beats taking place in the heart of the narrator; the effect of this torture device is that he thinks of a point when time will cease for him. Both the pit and the pendulum serve to build suspense in the story until the narrator's climactic rescue.