Hardly anything might seem more audacious than to deny that the arch of Constantine was built in honor of that emperor; yet the really amazing thing is our failure to attend to the numerous hints that this arch had existed long before Constantine. Artists and archaeologists have always been un-able to explain how an architect of the decadent age of Constantine could have given to this arch its marvellous proportions and silhouette, which set it above all other arches, even those of the golden age (Fig. 1). Historians have been puzzled by the silence of that early catalogue of the buildings at Rome, the Notitia, issued before Constantine's death (334 A.D.), which assigns to Constantine, apparently, only the Janus in the Forum Boarium. The same Notitia increases the mystery by speaking of an Arcus Novus on the Via Lata, which can only be the arch of Diocletian, dedicated in 303. If in 334 the arch of 303 was still the latest of triumphal arches, how could an arch have been built to Constantine in 315 ?