Few are those who, having come to Rome for a holiday, notice, in the Basilica of Saint Peter, a great disk in red porphyry, set in the pavement just past the door to the aisle. This is the famous "rota porphyretica" or wheel of coronation, that comes from the old Saint Peter’s, the ancient basilica built by Constantine. It was on this stone that stood the emperors when they were crowned by the popes. On this great wheel, opportunely preserved, knelt the great Carlo Magno, king of the Franks, the eve of Christmas of the year 800 when Pope Leo III (795-816) crowned him "Roman Emperor" (so was he acclaimed by all those present), placing the Imperial Diadem upon his head.
On that same wheel numerous other emperors were also crowned, among them Lotarian I, Ludvic II, Lambert of Spoleto, Berengar, the two Ottonians, Frederick Barbarossa and Frederick II.
We apologize but we were unable to find an image of the "rota" on the web, we swear you that, as soon as possible, we’ll take a picture by ourselves and we’ll put it on the blog immediately!