To allow a dome to rest above a square base, either of two devices was used: the squinch (an arch in each of the corners of a square base that transforms it into an octagon) or the pendentive.
"The most distinctive feature was the domed roof. To allow a dome to rest above a square Base, either of two devices was used: the squinch (an arch in each of the corners of a square Base that transforms it into an octagon) or the pendentive."
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