Having survived the depths of Hell, Dante and Virgil ascend out of the undergloom to the Mountain of Purgatory on the far side of the world.
The Mountain is on an island, the only land in the Southern Hemisphere, created by the displacement of rock which resulted when Satan's fall created Hell The classification of sin here is more psychological than that of the Inferno, being based on motives, rather than actions.
Florence's Guelphs split into factions around 1300: the White Guelphs and the Black Guelphs.
Dante was among the White Guelphs who were exiled in 1302 by the Lord-Mayor Cante de' Gabrielli di Gubbio, after troops under Charles of Valois entered the city, at the request of Pope Boniface VIII, who supported the Black Guelphs.
The core seven sins within Purgatory correspond to a moral scheme of love perverted, subdivided into three groups corresponding to excessive love (Lust, Gluttony, Greed), deficient love (Sloth), and malicious love (Wrath, Envy, Pride).
In central Italy's political struggle between Guelphs and Ghibellines, Dante was part of the Guelphs, who in general favored the Papacy over the Holy Roman Emperor.
Beatrice was a Florentine woman whom he had met in childhood and admired from afar in the mode of the then-fashionable courtly love tradition, which is highlighted in Dante's earlier work La Vita Nuova.
The structure of the three realms follows a common numerical pattern of 9 plus 1, for a total of 10: 9 circles of the Inferno, followed by Lucifer contained at its bottom; 9 rings of Mount Purgatory, followed by the Garden of Eden crowning its summit; and the 9 celestial bodies of Paradiso, followed by the Empyrean containing the very essence of God.
The Roman poet Virgil guides him through Hell and Purgatory; Beatrice, Dante's ideal woman, guides him through Heaven.
The Purgatorio is notable for demonstrating the medieval knowledge of a spherical Earth.
During the poem, Dante discusses the different stars visible in the southern hemisphere, the altered position of the sun, and the various timezones of the Earth.
Each sin's punishment in Inferno is a contrapasso, a symbolic instance of poetic justice; for example, in Canto XX, fortune-tellers and soothsayers must walk with their heads on backwards, unable to see what is ahead, because that was what they had tried to do in life: they had their faces twisted toward their haunches and found it necessary to walk backward, because they could not see ahead of them. and since he wanted so to see ahead, he looks behind and walks a backward path.
These three types of sin also provide the three main divisions of Dante's Hell: Upper Hell, outside the city of Dis, for the four sins of indulgence (lust, gluttony, avarice, anger); Circle 7 for the sins of violence; and Circles 8 and 9 for the sins of malice (fraud and treachery).
An initial canto, serving as an introduction to the poem and generally considered to be part of the first cantica, brings the total number of cantos to 100.