When Dante encounters Bertrand de Born in the Ninth Circle of hell, the troubadour exclaims:
Così s'osserva in me lo Contrapasso.
Longfellow translates this as "Thus is observed in me the counterpoise," which does not quite capture the point de Born is making. He has had his head cut off and is carrying it around the lowest depths of hell like a lantern. This is his punishment for setting father against son, when he persuaded the Young King Henry to rebel against his father, Henry II of England. Because he severed father from son, his own head is severed from his body.
Contrapasso means "suffering the opposite." However, in the case of Bertrand de Born, the punishment he suffers is not the opposite of his sin but similar to it. In the Inferno, therefore, contrapasso refers to any instance of poetic justice, in which the punishment fits the crime either because it is similar to or the opposite of the sin committed. One of the best-known examples of contapasso in the Inferno is that of the false prophets in the Eighth Circle of hell, who have their heads turned backwards so that they cannot see where they are going. This is because they claimed to be able to foresee the future (that is, what lay ahead) when they were alive.