Parallelism is a literary device in which the author repeats the same kind of grammatical structure multiple times in a row for effect. Poe makes vivid use of parallelism when he describes the pendulum’s steady, terrible descent. Three short paragraphs in a row begin with sentences constructed exactly the same way, and even beginning with exactly the same word: “Down.” He begins: “Down—steadily down it crept” (253). The next paragraph has: “Down—certainly, relentlessly down!” (254). In the third, we read: “Down—still unceasingly—still inevitably down!” (254)

These parallel sentences all share a basic pattern: the word “down,” an emphatic dash, and a smattering of tortured adverbs. But they also evolve, moving from periods to exclamation points, and from a single adverb to a whole series. That combination of repetition and intensification mirrors exactly what the sentences are describing: the relentless swing of the pendulum, which indeed becomes more and more fearful as it comes “down.” Notice, too, how Poe both begins and ends each of these sentences with the word “down,” evoking the back-and-forth motion of the pendulum even more precisely.