Giovanni di Paolo, 'Saints Fabian and Sebastian', about 1475

Saint Fabian, on the left, and Saint Sebastian tower above two tiny kneeling figures wearing black hooded robes with long white veils and holding objects that look like paddles. Certain religious groups used instruments like this for self-flagellation – the practice of whipping yourself as a punishment for sin.

Saint Fabian was pope in the third century and was killed by the Roman Emperor Decius. He is shown dressed in papal robes and tiara – the three-tiered crown. His white cope is woven with golden thread and studded with pearls. Saint Sebastian is shown naked but for a translucent loin cloth. He has curly blonde hair, wiry calf muscles, bony knees and a muscular abdomen. He is pierced by 20 arrows, each wound oozing droplets of blood. The vibrant red pigment of the droplets is one of the best-preserved elements of the picture. According to the Golden Legend, a thirteenth-century compilation of the lives of the saints, Saint Sebastian was tied to a post and shot by Emperor Diocletian’s soldiers: ‘They shot so many arrows into his body that he looked like a porcupine.’ He miraculously survived this torture but was later beaten to death.

Both saints were both thought to protect Christians against the plague and they share a feast day on 20 January – so they were often shown together. Sebastian converted many people to Christianity and in doing so they were cured of their diseases. Centuries later in the northern Italian town of Pavia, the citizens suffered a deadly plague which was stopped only when they built and dedicated an altar to Saint Sebastian.

During the eighteenth-century in Siena, the city in which this picture was made, a religious group called the Bianchi had an altar dedicated to Fabian and Sebastian, also hoping for protection against plague. So the kneeling figures we see could be members of the Bianchi, though they were renowned for wearing all white – but we just don't know for sure.

The original use of this picture is mysterious: there are no signs that it ever formed part of an altarpiece, and there is no evidence to suggest that it was ever attached to other panels. Independent panels were popular in Siena for private prayer but this panel is too large to have been used for that. It is almost a metre in height, and the frame is original except for at the bottom edge where it has been replaced.

The panel has been thinned down on the reverse and so it is possible that it was painted on both sides. If so, it may have been used in religious processions where both sides would have been visible at once. The damage to the lower edge – which led to it being replaced – might have been caused by a pole, inserted so the picture could be carried as a banner in processions.