Its length from east to west is accurately determined by the fact that the Via Appia, which runs from north-west to south-east from Casilinum to Calatia, turns due east very soon after passing the so-called Arch of Hadrian (a triumphal arch of brickwork, once faced with marble, with three openings, erected in honour of some emperor unknown), and continues to run in this direction for 1600 meters (6,000 ancient Oscan feet).