This type of structure is a tetrapylon with four arches and two passageways which intersect (in the form of a cross) in the center. Janus is both the name for this kind of structure as well as the name of the god associated with gates and doors. In this role Janus was also thought to represent beginnings. This belief comes from the idea that one must emerge through a gate or door before entering a new place. He is represented artistically with two opposite faces.
Niches with shell-shaped vaults once were articulated by columns on each side, now eroded or lost. Like earlier examples of this type of arch, Janus Quadrifrons originally had an additional story.