“Dante was a keen follower of Pythagoras, the sixth-century BC Greek philosopher and mathematician who sought to explain the world, both spiritual and material, by numbers. Pythagoras believed that the mathematical principles that underlay the universe, gave it harmony, literally a music of the spheres. Dante, in his great work, Divine Comedy, sought to create the divine song.
The key number for Dante was 11—the union of 5 and 5—and its multiples. The Inferno, Purgatorio, and Paradiso, have thirty-three cantos each, and the poem is written in hendecasyllabic rhyme (eleven syllables long). Dante twice provides dimensions of Hell, stating that that circumference of the ninth bolgia (ditch) in the Eighth Circle is 22 miles (miglia ventidue), and the tenth bolgia is 11 miles. There is nothing accidental about this mention of 11 and its multiple 22; twenty-two forms part of the well-known fraction 22/7 which expresses the Pythagorean value of pi.
Three and nine also figure prominently in Dante’s numerology. The three books of the Divine Comedy delineate the nine circles of Hell, the nine rings of Mount Purgatory and the nine celestial bodies of Paradise.”
Excerpted from: Rogerson, Barnaby. Rogerson’s Book of Numbers: The Culture of Numbers–from 1,001 Nights to the Seven Wonders of the World. New York: Picador, 2013.