Terza rima (third rhyme), also known as terzina Dantesca, is a style of poetry Dante created to write The Divine Comedy. No earlier examples are known. It’s believed he was influenced by the Provençal troubadours he so admired. They used a form of lyric poetry called sirventes or serventes (service song).
These songs were written from the POV of a sirvent (serviceman), and were usually parodies. They borrowed the metrical structure, melody, and many times even the actual rhymes of famous songs to take on controversial subjects (often current events). A sirventes more often than not was quite vitriolic.
Terza rima, then, is a three-line stanza with the rhyming pattern of ABA BCB CDC DED; i.e., the first and third lines rhyme, and the second line rhymes with the first and third lines of the next triplet. Each section of the poem concludes with a couplet or single line repeating the final tercet’s middle line’s rhyme. E.g., DED E, DED EE.
It’s known as chained rhyme because the second verse hooks each triplet onto the next like a chain. This style makes it easier to memorise than poems and songs with only two rhyming lines. (Though nowadays, many singers and rappers can’t even be bothered to rhyme anything, rhyme words with themselves, or just repeat lines.)
Dante’s one exception is Christ only rhyming with itself in Paradiso. He felt it would be unholy and blasphemous to associate any other words with that name.
This style of rhyming also makes it more difficult for copyists to steal the work, delete some lines, and embellish it with their own lines. The rhyme sequence would be interrupted if anything were taken away or added, and coming up with new rhymes to fit with the overall story and not look like piracy would be a really difficult task,
Terza rima employs the hendecasyllable (endecasillabo) structure, a verse of eleven syllables where the last accent falls on the tenth syllable. This too was influenced by the Provençal troubadours, corresponding to their décasyllabe.
Because of Dante’s acclaim, other Italian poets began using terza rima too. To this day, it remains the most popular metric structure of Italian poetry and song.
Terza rima is very difficult to naturally achieve in English due to the language’s more complex phonology and relative dearth of words to easily rhyme with one another. Even a lot of singers, poets, and rappers just using standard rhyming often employ forced rhyme schemes because the words at the end of those two consecutive lines sound nothing alike, and there were no other words they could think of.
English writers brave enough to use terza rima include Geoffrey Chaucer, Percy Bysshe Shelley, John Milton, Thomas Hardy, Lord Byron, W.H. Auden, T.S. Eliot, Robert Frost, and William Carlos Williams. Several translators of The Divine Comedy have also used this metric, among them Laurence Binyon, who did the version I first read. (And yes, it does often employ forced rhyme schemes.)
Translators using terza rima necessarily also take some liberties with the source text. To ensure the English lines all follow that style, there are frequent insertions of words and phrases that appear nowhere in the actual Italian. As pretty as they make it sound, it’s linguistically misleading.
…Tu lascerai ogne cosa diletta
più caramente; e questo è quello strale
che l’arco de lo essilio pria saetta.
Tu proverai sì come sa di sale
lo pane altrui, e come è duro calle
lo scendere e ‘l salir per l’altrui scale…
…You shall leave everything you love most:
this is the arrow that the bow of exile
shoots first. You are to know the bitter taste
of others’ bread, how salty it is, and know
how hard a path it is for one who goes
ascending and descending others’ stairs…
(Paradiso, Canto XVII, 55–60)