Purgatory has 33 cantos in terza rima and recounts the climb of a mountain in three parts: Ante-Purgatory, at the base, where those who came late wait; the seven ledges, where each sin is purged; and the Earthly Paradise, at the summit. A gate guarded by an angel, in canto IX, separates the first part from the second.
In Ante-Purgatory (cantos I to VIII), the souls wait a time proportional to the delay they had in life, unless the prayers of the living shorten it. At the gate, the angel marks seven P's on Dante's forehead, one for each sin to be washed away. The ledges come in this order: pride (X to XII), envy (XIII and XIV), wrath (XV and XVI), sloth (XVII and XVIII), avarice (XIX to XXI), gluttony (XXII to XXIV) and lust (XXV and XXVI). At the summit (XXVII to XXXIII) are the wall of fire, the forest, the river of two waters and the procession.
The logic of the climb is the doctrine of love from canto XVII. Three ledges cleanse love of harm to a neighbor, one cleanses lukewarm love of the good, and three cleanse excessive love of lesser goods. There is also a rule of the place that governs time: one does not climb at night, because darkness hampers the will with its inability. For this reason the cantica counts the climb in days and nights, and each night's halt becomes an occasion for conversation.