Here's how it works:
* A lines rhyme with each other (e.g., "Once upon a midnight dreary, while I pondered, weak and weary").
* B lines rhyme with each other (e.g., "Deep into that darkness peering, long I stood there wondering, fearing,").
This pattern continues throughout the poem, creating a strong sense of repetition and predictability that mirrors the speaker's descent into madness.
Poe was a master of using form and sound to evoke specific emotions. The consistent, predictable rhyme scheme, coupled with the repetition of the raven's "Nevermore", helps emphasize the speaker's growing despair and the relentlessness of his grief.