In the two mosaic panels on the sides of the conch has preserved the oldest representation of the Banner of the Papal States, They depict Christ with the Apostles in the center; Christ with Constantine and Pope Sylvester I on the left; and St Peter, Pope Leo III and Charlemagne on the right. Pope Leo III has a square nimbus, showing that he was alive when it was made. The mosaic has been dated to the year 800, when Charlemagne was crowned in Rome.