Along the nave, and the first aisles either side of the cental nave, above the arcades, there are roundels containing portrait mosaics of all the popes from Peter to Francis. Traditionally the series was initiated by Pope Leo the Great (440-461). Those portraits were destroyed in the fire in 1823, apart from forty-one kept in the museum. Pope Pius IX (1846-1878) restarted the project in 1847, specifying mosaics instead of frescoes, and since then it has been updated whenever a new pope is elected. There are currently seven spaces available.