The significance of the title of Edgar Allan Poe's gothic 1842 classic "The Pit and the Pendulum" is quite elementary. During the Spanish Inquisition, an unnamed narrator is put on trial, convicted and put in jail. No ordinary jail cell, the narrator discovers he is in some kind of torture chamber where he will presumably be killed. A giant pendulum like one would find on a grandfather clock is swinging from above and descends slowly toward him. As he had earlier gazed upon a clock with a swinging pendulum, and that there is a picture of Father Time on the wall, the meaning of the giant but deadly pendulum descending from above is clear -- to the extent that allegories conjured up by Poe can ever be clear.
Also within the cell is a pit, the thought of falling into which terrifies the narrator. As he begins to lose his balance and fall into the pit, he is rescued.
Poe's horror was more psychological than real. His stories built suspense by increasing the terror in the mind of an individual. No death was necessary, the fear of a horrific demise was sufficient to frighten the reader. The pit and the pendulum were two dramatic devices Poe used because one, the pendulum, represents time, which is used to build suspense, and he used the other, the pit, because of the existential terror most people fear of the unknown. How deep was the pit, or what was in it, we never know. We only know we don't want to fall into it.