Red porphyry disc (disque de porphyre rouge) in the nave on which Charlemagne and other Emperors were crowned.The great disc of red porphyry stands out against the marble paving. It comes from the old basilica; where it was located near the main altar. Kneeling on it; the Emperor Charlemagne was crowned Emperor by Pope Leo III; at Christmas in the year 800. Another 21 emperors subsequently knelt on this same disc to receive the crown of the Holy Roman Empire from the Pope's hands.( Saint Peter's basilica; basilique Saint-Pierre; Petersdom. )Vatican 2019