To answer this question while also offering a visual, I will label the rhyme scheme for an actual stanza of the poem.
Once upon a midnight dreary, while I pondered weak and weary, A
Over many a quaint and curious volume of forgotten lore, B
While I nodded, nearly napping, suddenly there came a tapping, C
As of some one gently rapping, rapping at my chamber door. B
`'Tis some visitor,' I muttered, `tapping at my chamber door - B
Only this, and nothing more.' B