Arch of Janus

The Arch of Janus is the only quadrifrons triumphal arch preserved in Rome. It was set up in the early 4th century AD at a crossroads at the northeastern limit of the Forum Boarium, close to the Velabrum, over the Cloaca Maxima drain that went from the Forum to the River Tiber.

The significance of the arch is poorly understood: it is thought to have been a boundary marker rather than a triumphal arch. An alternative view is that it was built to provide shelter for the traders at the Forum Boarium cattle market. Some researchers believe it was dedicated to Constantine I or Constantius II and was known as the Arch of the deified Constantine. It is 16 meters high and 12 meters wide; it was originally higher, but the attic storey was removed in 1830 in the erroneous belief it was all medieval.