Janus is a god with two faces according to Roman mythology. One face of Janus faces to the right, the other face to the left. Janus is the city's guardian. Janus follows events in the city with one side of her face. Janus, with the other face, checks the danger that the city may come from outside.
According to another idea, Janus' face represents concrete concepts on one side, and abstract concepts on the other side. The roots of the month of January are also from Janus.