Squinch

A squinch is a small arch, corbelled niche, or nested sequence of miniature arches built diagonally across each interior corner of a square chamber, converting the square into an octagon capable of carrying a circular or polygonal dome ring above it. Without some such device, a dome cannot rest directly on a square base, because a circle's circumference does not meet a square's corners; the squinch solves the problem by bridging each corner with enough masonry to approximate the missing arc, typically in two or three stepped tiers that visually announce the transition from straight wall to curved dome even as they perform the load-bearing work of the change.

In mosque architecture the squinch appears wherever a dome caps a square chamber, commonly over the bay directly in front of the mihrab, marking that space as the building's ritual focus through the visual drama of the transition zone as much as through the dome itself. Squinches are typically the most heavily decorated structural element in a domed chamber, since their stepped surfaces catch light and shadow in ways flat walls do not, and by the later medieval period many mosque builders replaced the plain squinch with the far more elaborate muqarnas, a related device built from many small nested cells rather than a few large arched tiers.

The squinch predates Islamic architecture by centuries and was never exclusive to it. Sasanian Persian fire temples used corbelled squinches to carry domes over square sanctuaries well before the seventh century, and Byzantine church architecture developed pendentives, a related but geometrically distinct solution using concave triangular sections rather than corner arches, to solve the same square-to-dome problem for buildings such as the sixth century Hagia Sophia. Early Islamic builders, working across formerly Sasanian and Byzantine territory, inherited both devices and used them somewhat interchangeably at first, with squinches more common in the eastern Islamic lands closer to the Persian tradition and pendentives more common where Byzantine building practice was strongest.

Over time squinches became the dominant device in Iranian, Central Asian and much of the wider Islamic world, developing from plain corbelled arches into increasingly elaborate multi-tiered and eventually muqarnas forms, while pendentives remained the standard solution in Byzantine and later Ottoman domed architecture, itself heavily influenced by Hagia Sophia. The two devices therefore sit on either side of a real historical boundary, Persian and Byzantine respectively, that Islamic building inherited whole rather than invented, and a single tradition's later choice between them often signals which of the two antecedent building cultures most shaped its regional school.