The Arch of Janus Quadrifrons (Arcus Constantini), a quadrifrons triumphal arch, was built in the 4th century on top of the Cloaca Maxima. The name "Arch of Janus" came later, perhaps in the Renaissance. The Arch of Janus is located at the Velabrum (a commercial area on the land between the northwest slope of the Palatine and the Capitoline) at the northeast corner of the Forum Boarium. It was built of concrete covered with white marble 12 sq. m x 16 m high. The arches are 10.6x5.7m.
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