The Cambridge Companion to Dante, ch. 10 (Ferrante, "A poetics of chaos and harmony): 01
This kind of symmetry and balance is characteristic only of Purgatorio, and is also to be found in the symmetrical patterns in the structure of the canticle and of each terrace (see chart 3).

The Cambridge Companion to Dante, ch. 10 (Ferrante, "A poetics of chaos and harmony): 02
Clear endings or new beginnings in Inferno:; cantos 1, 2, 3, 4, 5, 12, 17, 18, 19, 20, 23, 27, 31, 33; endings that look ahead: cantos G. 7, 9, 10, 11, 16. Distinct endings in Pugrgatorio: cantos 1, 12, 13, 14, 24, 27. Noncontinuous cantos in Paradiso: 7, 9, 20, 29. Barolini discusses transitions and endings in an appendix of the forthcoming The Undivine Comedy; she finds only eleven cantos in Inferno which have no clear transition, whereas Paradiso, in reverse, has clear transitional endings in eleven. Though we classify beginnings and endings differently, our findings are similar.

The Cambridge Companion to Dante, ch. 10 (Ferrante, "A poetics of chaos and harmony): 03
The Latin consists of a few phrases spoken by Virgil: "sub Iulio" (1, 70), and "Vexilla regis prodeunt inferni" (34, 1, which parodies a hymn), Brunetto Latini ("ab antico," 15, 62, part of his snobbery about the pure Florentines), Guido da Montefeltro ("quare," 27, 72, a suggestion perhaps of his logical but flawed view), and Dante ("Miserere" to Virgil, 1, 65, a cry for help, and "suo loco," 18, 6, as he begins to describe the careful plan of the Malebolge).

The Cambridge Companion to Dante, ch. 10 (Ferrante, "A poetics of chaos and harmony): 04
Similarly, "scala," the stairway or ladder of contemplation, moves to "cala," the descent which accommodates the human "ala," wing or capacity, Paradiso 22, and Purgatorio 3, where the worst ascent on earth seems a stairway compared to the foot of the cliff in Purgatory, which must slope downward for Dante and Virgil who climb without wings.

The Cambridge Companion to Dante, ch. 10 (Ferrante, "A poetics of chaos and harmony): 05
Canto 29 has the largest number of rhymes linked by consonance or assonance that I have found, 96 of the 145 lines, or 66 percent, if my count is correct, and that does not include some 20 more linked by alliteration. Bernard's description of the rose in canto 31 is also filled with linked rhymes, 89 of IS 1, or 59 percent, as well as nearly identical but non-rhyming rhymes ("scanno / scanni, Agusta / aggiusta / gusto / gusta"), nearly identical rhymes ("discese / distese, vetusto / venusto, chiavi / clavi"), the identical Cristo rhyme, a five-rhyme and a four-rhyme.