squinch

English squinch is a variant of 'scunch' or 'sconcheon' (an Old English/Old French word for the interior face of a doorway jamb or the corner masonry). The squinch is the simplest of the two methods (the other being the pendentive) for transitioning from a square room to a circular dome: a small arch, often filled with corbelled masonry, is placed across each corner of the square, converting it to an octagon, then a sixteen-sided polygon, then the circle of the dome base.

The squinch is older than the pendentive — it appears in Sassanid Persian architecture (3rd-7th century CE) and in early Islamic architecture. The Sassanid palace of Sarvestan (c. 400 CE) uses squinch arches to support its dome. The technique was known in the Eastern Mediterranean world before the Byzantines perfected the pendentive.

The word squinch entered English architectural vocabulary in the 18th century, when antiquarians and architects began studying the buildings of the past systematically. The term codified something that had been built for centuries without a consistent English name. Thomas Rickman's 1817 Attempt to Discriminate the Styles of English Architecture introduced many architectural terms that standardized the vocabulary.

The squinch and pendentive are often confused — both solve the same problem, but the squinch is corner-arch (creating an octagonal intermediate) while the pendentive is a concave triangular sail. The pendentive is considered the more sophisticated solution; the squinch the simpler. Both are still used. The word squinch, with its cheerful vowel sounds, has survived in a vocabulary where most terms sound Latin and grave.