A large four-sided marble arch, which stands between the Velabrum and the Forum Boarium, can be identified with an « arcus divi Constantini » mentioned in the Regionaries of the Constantinian period, in Region XI. Fragments of the dedicatory inscription from the arch are walled in the facade and interior of the Church of S. Giorgio in Velabro and seem to indicate that it was erected in honor of Constantine, probably by Constantius II around A.D. 256 when he visited Rome.
The conventional name of Arch of Janus derives from the term Ianus (patron god of gateways) used to designate covered passageways, arcades and arches.
It measures 12 meters per side and is 16 meters high. The four piers, faced with marble slabs (in part reused), stand on molded plinths. Above the plinth, the two outer faces of each pier have two rows of semi-circular niches with shell-shaped conchas, which held statues and were separated by a cornice. Originally they must have been framed by small columns which in the lower row rested on the cornice of the plinth, and in the upper row rested on the cornice at the height of the opening. The round arches lead into barrel vaults which at their crossing form a cross vault with brick ribbing. The attic, of brick faced with marble, was torn down in 1827 as a medieval addition.
The predilection for a facade of niches framed by small columns on corbels is typical of the 4th century. The Arch of Janus therefore can be compared with the north apse of the Basilica of Maxentius, Maxentius’ restructuration in 307 of the Temple of Venus in Rome, with the facade of Diocletian’s palace in Spalato and with the changes made by Diocletian inside the Curia.
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