This altarpiece fragment depicts the contemporary saint St Peter Martyr, who died in 1252 and was canonised in 1253. Recognisable by his skull split in two by an axe and his martyr's palm, this Dominican was assassinated by a Cathar. A heresy against which he fought fervently, Catharism discerned two gods, one good and one bad, and attacked the Catholic hierarchy. It developed in Occitania, but also in northern Italy, and was one of the major opponents of the papacy in the 13th century. The Dominicans formed part of the Inquisition. Founded in 1199, this court was responsible for the conversion or condemnation of heretics.

Taddeo di Bartolo gives the saint a very sweet face with a penetrating gaze. The mantle with its soft lines reinforces the calm demeanour of this man indifferent to his martyrdom.