The Arch of Constantine in Rome marks the passing of the pre-Christian era in architectural terms, recapitulating imperial traditions while at the same time heralding a new consciousness. It pioneered modes of design that exploited recycled elements for the sake of effects and motives quite beyond purely pragmatic considerations. Long the subject of controversy, the monument is today the focus of a scholarly quarrel over the possibility that its superstructure once belonged to an earlier arch on the same site. This study refutes this hypothesis on the basis of considerations of technique and design, showing instead that its composition depended on the emulation of the nearby Arch of Septimius Severus. The connection between the two buildings is indeed as direct as that between Trajan's Column and its full-scale "copy," that of Marcus Aurelius, and it is possible to unravel the rationale behind the transformation of one arch into the other. Since the composition of Constantine's was in this way effectively resolved, the architect could concentrate on the adaptations necessary for accommodating the various sets of recycled components. And despite their heterogeneous character, the outcome was a project of singular coherence in terms of proportion and geometry. It was the product of a unitarian conception that promoted Constantine's ideological program in the realm of urban design, imperial iconography, and political and religious intent.