The Romans didn't invent the arch - it was known but not much used in Greek architecture - but they were the first to master the use of multiple arches, or vaults. From this, they invented the Roman groin vault - two barrel vaults set at right-angles - which represented a revolutionary improvement on the old Greek post-and-lintel method, as it enabled architects to support far heavier loads and to span much wider openings. The Romans also made frequent use of the semicircular arch, typically without resorting to mortar: relying instead on the precision of their stonework.
Arches and vaults played a critical role in the erection of buildings like the Baths of Diocletian and the Baths of Caracalla, the Basilica of Maxentius and the Colosseum. The arch was also an essential component in the building of bridges, exemplified by the Pont du Gard and the bridge at Merida, and aqueducts, exemplified by the one at Segovia, and also the Aqua Claudia and Anio Novus in Rome itself.