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Squinch and pendentive: from the 5th century AD

Both the squinch, developed in about the 5th century AD in the Persian empire, and the pendentive, possibly deriving from Syria, are ways of resting a dome upon a square base.

The squinch is the simpler of the two. Fixed in the angle of two walls, rather like a swallow's nest in a corner of a courtyard, it provides on its top outer edge a support for the dome. Four squinches, one at each corner, effectively turn a square into an octagon - a shape on which it is possible to construct a dome.

The squinch has to be achieved by building a short bridge across each corner of a square structure, either by a system of corbelling (with projecting courses of stone or brick) or by constructing a small arch.

The pendentive is a much more sophisticated concept, simpler in appearance but more complex in its geometry. It links the horizontal curve of the dome's base directly to the vertical curves of the four supporting arches. By this means it becomes possible to place a dome above four open arches without any intervening rectangular structure. The pendentive opens the way to the lightness and grace characteristic of domed architecture.

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