A pattern at the center of Purgatorio

It seems that Cantos 14 through 20 contain another architectonic pattern that remained unnoticed for centuries, until Charles Singleton pointed to it in 1965: The symmetrical canto lengths surrounding Canto 17 give us a structural basis for calling that canto the center of the entire Commedia. (This makes more sense if Canto I of the Inferno is viewed as a prologue to the entire poem, i.e., 1 + 33 + 33 + 33 = 100.)

This pattern involves a mirroring of numbers of lines around the center:

Canto 14: 151 lines

Canto 15: 145 lines
Canto 16: 145 lines

Canto 17: 139 lines

Canto 18: 145 lines
Canto 19: 145 lines

Canto 20: 151 lines

There is an additional pair of "pendants" involving the phrase "libero arbitrio" in cantos 16 and 18.

It wasn't until the 20th century that scholars began to analyze the ring structure in Homer's Iliad - yet Dante seems to have created his own "mini-ring" in his epic without having access either to Homer or their scholarship.