This article proposes that nearly all of the sculpted frieze of the Arch of Constantine in Rome, generally regarded as Constantinian, derives from a triumphal monument of Diocletian commissioned shortly after his Vicennalia in 303 CE. The basis of the argument is the sculptural technique evinced by the frieze, especially the separately-worked heads of the emperor in four of the frieze slabs, together with the missing legs and feet of several of the other figures. These anomalies suggest that much of the frieze was spoliated from another monument that had honored a different man; Diocletian is the only emperor whose career fits the iconography. A Diocletianic date for most of the frieze blocks necessitates a reconsideration of long-standing interpretations of the spolia on the arch and, in turn, its historiography.
