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Arch, a constructional feature employed to span openings or cover over space, and built with stones or bricks, so arranged as to exercise mutual pressure, and thereby to support a superstructure. Arches are of several forms, the simplest of which are the semicircular (a) and the segmental (b), both of which are struck from one centre. These forms are found in early Egyptian architecture, and the semicircular arch is a characteristic feature of the Assyrian, Etruscan, Roman, Byzantine, and Romanesque styles.

The pointed arch is struck from two centres, the two curves meeting in a point at the top. When the centres coincide with the sides of the arch, it is called equilateral. When they are without the curve, the arch is called lancet.

The pointed arch is a stronger form than the semicircular, and its earliest example is found in the vaulted drains at Nimroud in Assyria. It is a characteristic feature of the Gothic or Pointed styles, and is supposed to have been derived from Saracenic examples in Syria and Egypt, where it was employed as early as the eighth and ninth centuries. In the fifteenth century, in English Gothic, an arch was employed which is struck from four centres, and is known as the four-centred or Tudor arch. About the same period was used a four-centred arch called the ogee, and of which two of the centres are within the curve, and two above it. This arch is characteristic of late French Gothic architecture known as "flamboyant;" it is found occasionally in English architecture, and is a well-known feature of Venetian Gothic. In French flamboyant architecture of late fifteenth century work there is found also a three-centred arch.

The horseshoe arch is a semicircular arch, the curve of which is carried down below the centre. This arch is characteristic of Moorish work in Spain, Morocco, and Tunis. In Saracenic architecture in Egypt and Syria the arches are sometimes horseshoe and pointed. The earliest example known of the horseshoe arch is found in Persia.

Besides these arches there are others of a more decorative form called foiled arches: they are known as trefoil and cinquefoil, according to the number of the foils; the junction of two foils, viz. the point where they meet, is called a cusp. Sometimes a complete opening is formed with foils, the distinguishing terms being as before, trefoil, quatrefoil, cinquefoil, sexfoil. Foiled arches are found in Western Europe employed from the thirteenth century to the fifteenth century. They were also characteristic features of the Moorish style, being found in the Great Mosque at Cordova, and in the Alhambra.

An ordinary arch is built on what is called a centre, framed in timber to support the stones of the arch until they are all in position. The blocks of a true arch are of a wedge-shaped form, and are called arch-stones or voussoirs. The lowest block on which the arch rests is called the springer, and its upper surface is known as the skew-back. The topmost stone is called the keystone, and is the last inserted. In true Gothic arches there is no keystone, the junction of the two sides being a vertical line.

The inner surface of the arch is called the soffit or intrados, the outer or upper surface the extrados. That portion of the arch which lies between the springer and the keystone is called the haunches. The portion of wall above the arch on each side is called the spandril.