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An arch is a curved structure capable of spanning a space while supporting significant weight (e.g. a doorway in a stone wall). The arch was developed in Ancient Greece and later refined in Ancient Rome. The arch became an important technique in cathedral building and is still used today in some modern structures as for example in bridges.

It is significant because it converts tensile stresses in spanning structural members into compression stresses only. Stone is weak in tension and cannot span signficant distances without collapsing under its own weight. By configuring it into an arch, signficant spans can be achieved.

Arches were used by the Egyptian, Babylonian, Greek and Assyrian civilizations for underground structures such as drains and vaults, but the ancient Romans were the first to use them widely above ground. The so-called Roman arch is semicircular, and built from an odd number of arch bricks (in modern architectural parlance, these are called voussoirs). The capstone or keystone is the topmost stone in the arch. This 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 semicircular arch can be flattened to make an elliptical arch. The horseshoe arch is based on the semicircular arch, but its lower ends are extended further round the circle until they start to converge. It was used in Islamic architecture, as in the Great Mosque of Damascus and in later Moorish buildings. It was used for decoration rather than for strength. The semicircular arch was followed in Europe by the pointed Gothic arch, whose centreline more closely followed the forces of compression and which was therefore stronger. This design had been used by the Assyrians as early as 722 BC. The parabolic and catenary arches are now known to be the theoretically strongest forms.

A dome is a three-dimensional application of the arch. Igloos are notable early structures making use of domes.

The arch was used in some bridges in China since the Song dynasty.

A special form of the arch is the triumphal arch[?], usually built to celebrate a victory in war. The most famous example of this is the Arc de Triomphe in Paris, France.

Natural rock formations may also be referred to as 'arches'. These natural arches are formed by erosion rather than being carved or constructed by man. See Arches National Park for examples.

The "Golden Arches" form the logo of McDonald's, often put on high poles to mark the restaurant.

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