The Arch of Janus is the only quadrifrons triumphal arch preserved in Rome, across a crossroads in the Velabrum-Forum Boarium. It was built in the early 4th century, using spolia, possibly in honour of Constantine I or Constantius II. Its current name probably dates from the Renaissance or later, and was not used to describe it in classical antiquity. The name is derived from the structure's four-fronted, four-arched configuration; relating this to the four-faced version of Janus (Ianus Quadrifons), as well as to actual Janus-related structures mentioned in historic descriptions of ancient Rome.