The Romans acquired much of their engineering skill from the Etruscans and took inspiration from Etruscan and Asian models for the semicircular arch. From them the Romans learned the use of the keystone arch, which allowed them to build extremely strong and durable structures. Many of these engineering and architectural projects are still standing. Some are still in use after two thousand years, like this bridge in Spain. Early Roman architects were influenced by Greek pillar and architrave construction. But the Greek design was limited in its abilities to cover large distances and bear heavy loads without falling from its own weight. The post and lintel construction includes a flat piece of stone that bridges a gap between two vertical supports. The upright and lintel supports have a defect. When a heavy weight is placed in the center of the span, the stone can be placed under undue stress and can crack in the middle. The Romans solved this problem by using a type of construction called a keystone voussoir arch. The engineering principle of the arch is quite simple. The circle is the strongest structural shape. The bow is only half of this perfect shape. To create a voussoir arch, tapered blocks of stone were cut and then arranged as in the diagram to the right. It was then raised at the ends. The ends rested on pillars made of blocks of stone or brick cemented together with pozzolana cement in the typical Roman arch bridge. Cement is named after the local mountain where the cement ingredients came from. The weight of the stone and concrete of the bridge itself compressed the tapering stones together, making the arch an extremely strong structure. During construction the ashlars were supported by a temporary wooden frame until the keystone was inserted. The Roman invention of the arch allowed architects to build larger structures than ever before. The extension of the arch idea led to the development of domes, niches and soffits. Arches and its derivatives were also used in triumphal arches, aqueducts, bridges, houses, theaters, exedra, nymphaeum, basilicas, baths and temples.