The Paradiso begins at the top of Mount Purgatory, at noon on the Wednesday after Easter. After ascending through the sphere of fire believed to exist in the earth's upper atmosphere (Canto I), Beatrice guides Dante through the nine celestial spheres of Heaven, to the Empyrean, which is the abode of God. The nine spheres are concentric, as in the standard medieval geocentric model of cosmology, which was derived from Ptolemy. The Empyrean is non-material. As with his Purgatory, the structure of Dante's Heaven is therefore of the form 9+1=10, with one of the ten regions different in nature from the other nine.
During the course of his journey, Dante meets and converses with several blessed souls. He is careful to say that these all actually live in bliss with God in the Empyrean:
"But all those souls grace the Empyrean; and each of them has gentle life though some sense the Eternal Spirit more, some less."
However, for Dante's benefit (and the benefit of his readers), he is "as a sign" shown various souls in planetary and stellar spheres that have some appropriate connotation.
While the structures of the Inferno and Purgatorio were based around different classifications of sin, the structure of the Paradiso is based on the four cardinal virtues (Prudence, Justice, Temperance, and Fortitude) and the three theological virtues (Faith, Hope, and Charity).