An arch is a structure that spans a space while supporting weight (e.g. a doorway in a stone wall). Arches appeared as early as the 2nd millennium BC in Mesopotamian brick architecture, but their systematic use started with the Ancient Romans who were the first to apply the technique to a wide range of structures.

History

Arches were known by the Mesopotamian Urartian, Harappan, Egyptian, Babylonian, Greek and Assyrian civilizations, but their use was infrequent and mostly confined to underground structures such as drains where the problem of lateral thrust is greatly diminished. The oldest arched city gate in the world, eight feet wide, was found in Ashkelon, Israel, and is dated to the middle bronze age.

The ancient Romans learned the arch from the Etruscans, refined it and were the first builders to tap its full potential for above ground buildings:

The Romans were the first builders in Europe, perhaps the first in the world, fully to appreciate the advantages of the arch, the vault and the dome.Robertson, D.S.: Greek and Roman Architecture, 2nd edn., Cambridge 1943, p.231

The Roman arch is semicircular, and built from an odd number of arch bricks (called voussoirs). An odd number of bricks is required for there to be a capstone or keystone, the topmost stone in the arch. The Roman arch's shape is the simplest to build, but not the strongest. There is a tendency for the sides to bulge outwards, which must be counteracted by an added weight of masonry to push them inwards. The Romans used this type of semicircular arch freely in many of their secular structures such as aqueducts, palaces and amphitheaters.Fact: date=December 2007

The semicircular arch was followed in Europe by the pointed Gothic arch or ogive (derived from the Islamic pointed arch in Moorish Spain), whose centreline more closely followed the forces of compression and which was therefore stronger. The semicircular arch can be flattened to make an elliptical arch as in the Ponte Santa Trinita. The parabolic and catenary arches are now known to be the theoretically strongest forms. Parabolic arches were introduced in construction by the Spanish architect Antoni Gaudí, who admired the structural system of Gothic style, but for the buttresses, which he termed “architectural crutches”. The catenary and parabolic arches carry all horizontal thrust to the foundation and so do not need additional elements.