Edgar Allan Poe wrote an essay called “The Philosophy of Composition,” which was published in Graham’s Magazine in April 1846. In the essay, Poe gives a detailed account of how he wrote his poem "The Raven," discussing the tone, setting, effect, beauty, and sound of the poem and briefly touching on the structure of the poem.
The former [rhythm] is trochaic — the latter [meter] is octametre acatalectic, alternating with heptameter catalectic repeated in the refrain of the fifth verse, and terminating with tetrameter catalectic. Less pedantically — the feet employed throughout (trochees) consist of a long syllable followed by a short [“Once up-on a mid-night drear-y…”]: the first line of the stanza consists of eight of these feet — the second of seven and a half (in effect two-thirds) — the third of eight — the fourth of seven and a half — the fifth the same — the sixth three and a half.
A catalectic (meaning "headless") structure is Poe's technique of omitting the final syllable in the trochees on lines 2, 4, 5, and 6 (which all rhyme), which emphasizes these lines by letting the accented syllable stand alone. Acatalectic means that lines 1 and 3 do contain a final, unaccented syllable.
Poe claims for himself the originality of this particular structure.
Now, each of these lines, taken individually, has been employed before, and what originality the “Raven” has, is in their combination into stanza; nothing even remotely approaching this combination has ever been attempted. The effect of this originality of combination is aided by other unusual, and some altogether novel effects, arising from an extension of the application of the principles of rhyme and alliteration.
Poe generally maintains an ABCBBB rhyme scheme throughout the eighteen stanzas, but the poem is significant because it has a unique pattern of end rhymes and internal rhymes, which make the rhyme scheme more specifically AA,B,CC,CB,B,B when accounting for end rhymes and internal rhymes.
End rhyme consists of rhyming words at the end of each line. In "The Raven," the end rhymes in lines 1 and 3 alternate "A" and "C," and in lines 2, 4, 5, and 6 the rhyme is always "B": "Lenore," "door," "more", "evermore," "Nevermore," and so on, always ending in "-or."
Lines 1 and 3 of each stanza have no corresponding lines with end rhymes, but they have internal rhymes instead. "Dreary" rhymes with "weary" in line 1, "napping" rhymes with "tapping" in line 3, and so on throughout the poem.
In addition, there is an internal rhyme in line 4, which rhymes with the end rhyme in line 3.
While I nodded, nearly napping, suddenly there came a tapping,
As of some one gently rapping, rapping at my chamber door.
The poem also makes extensive use of alliteration—"weak and weary," "quaint and curious," " silken sad uncertain rustling," "Doubting, dreaming dreams."
"The Raven" is truly a marvel of poetic construction.
It's interesting to note that Poe at first considered a parrot as the voice in the poem, but decided on a raven instead.
Here, then, immediately arose the idea of a non -reasoning creature capable of speech; and, very naturally, a parrot, in the first instance, suggested itself, but was superseded forthwith by a Raven, as equally capable of speech, and infinitely more in keeping with the intended tone.