Canto IV Virgil proceeds to guide Dante through the nine circles of Hell. The circles are concentric, representing a gradual increase in wickedness, and culminating at the centre of the earth, where Satan is held in bondage. The sinners of each circle are punished for eternity in a fashion fitting their crimes: each punishment is a contrapasso, a symbolic instance of poetic justice. For example, later in the poem, Dante and Virgil encounter fortune-tellers who must walk forward with their heads on backward, unable to see what is ahead, because they tried to see the future through forbidden means.