Arch is a curved structure that supports or strengthens a building. Almost all arches span openings and support weight above them. Others are enclosed in walls.
Most arches are made of stone, brick, concrete, or steel. Arches of stone or brick consist of wedge-shaped blocks called voussoirs. During the construction of most such arches, the blocks are supported by a wooden frame. The last block to be inserted is the keystone, the center stone at the top. The pressure of each side of the arc against the keystone supports the arch when the frame is removed. In addition, the arch is supported on both sides by masonry or by other arches to keep it from collapsing under the weight above.
The first people to fully utilize the arch were architects of ancient Rome. During the 300's B.C., they began to use semicircular arches to build aqueducts and bridges. Later, they also constructed triumphal arches to honor their leaders. Pointed arches were developed during the Middle Ages. Medieval architects arranged arches in rows to form passageways called arcades. They also built arched roofs called vaults. Arches shaped like horseshoes are common in Islamic architecture.