Commentary Purg XVIII 89-90

These penitents, as opposed to the wrathful, move in the same counterclockwise direction that is followed by Dante and Virgil.  It would seem that, while the movement of Dante to the right is morally determined, that of the penitents is not, is rather the result of aesthetic considerations.  Review of the various groups that are in motion suggests that such is the case. Below the gate, we see that the excommunicate are moving rightwards ([Purg III 59]), while the late-repentant seem to be heading to the left ([Purg V 23]).  On the terraces, the prideful seem to be moving right ([Purg X 100]), while the wrathful are headed left ([Purg XV 142]).  Here in Sloth the penitents come from behind Virgil and Dante but are moving in the same direction (verse 90).  Farther up the mountain the penitent gluttons are also moving counterclockwise ([Purg XXIII 19-20]), while the two distinct bands of the lustful proceed in two different directions ([Purg XXVI 29]).