<P> The flying buttress (arc - boutant, arch buttress) is a specific form of buttress composed of an arched structure that extends from the upper portion of a wall to a pier of great mass, in order to convey to the ground the lateral forces that push a wall outwards, which are forces that arise from vaulted ceilings of stone and from wind - loading on roofs . </P> <P> The defining, functional characteristic of a flying buttress is that it is not in contact with the wall it supports, like a traditional buttress, and so transmits the lateral forces across the span of intervening space between the wall and the pier . To provide lateral support, flying - buttress systems are composed of two parts: (i) a massive pier, a vertical block of masonry situated away from the building wall, and (ii) an arch that bridges the span between the pier and the wall--either a segmental arch or a quadrant arch--the flyer of the flying buttress . </P> <P> As a lateral - support system, the flying buttress was developed during late antiquity and later flourished during the Gothic period (12th--16th c .) of architecture . Ancient examples of the flying buttress can be found on the Basilica of San Vitale in Ravenna and on the Rotunda of Galerius in Thessaloniki . The architectural - element precursors of the medieval flying buttress derive from Byzantine architecture and Romanesque architecture, in the design of churches, such as Durham Cathedral, where arches transmit the lateral thrust of the stone vault over the aisles; the arches were hidden under the gallery roof, and transmitted the lateral forces to the massive, outer walls . By the decade of 1160, architects in the Île - de-France region employed similar lateral - support systems that featured longer arches of finer design, which run from the outer surface of the clerestory wall, over the roof of the side aisles (hence are visible from the outside) to meet a heavy, vertical buttress rising above the top of the outer wall . </P>

What is the purpose of a flying buttress
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