Squinch

A squinch, tromp in Turkish, is a transition element that bridges the upper corner of a square room diagonally. It can take the form of a half-cone, a niche or a series of stepped arches. With one in each corner the square becomes an octagon, and the dome or its drum sits on that octagon. It is generally regarded as an older solution than the pendentive and is widespread in Iranian and Anatolian architecture.

How the load travels

A squinch works like a small arch or vault spanning the corner between two walls, carrying the dome load into those walls. Over a plain square the dome would be unsupported at the corners; the squinches close that gap so the dome bears on eight points. The small difference between octagon and circle is easily taken up at the dome springing or in the drum.

• Conical squinch: a half-cone set into the corner
• Niche squinch: a semi-dome or niche-shaped corner
• Stepped squinch: small arches corbelled one above another
• Muqarnas squinch: a corner surface filled with muqarnas

Squinch versus pendentive

A pendentive moves from square to circle in one continuous spherical surface. A squinch creates an intermediate octagon, so the transition is more visible and angular. In Seljuk buildings, especially tomb towers and some madrasa domes, squinches are often enriched with muqarnas.

Survey and 3D modeling

The real geometry of a squinch is often hidden under plaster and ornament. Measure the niche depth, arch span and octagon side lengths separately. In a 3D model, building the squinch as a distinct part of the octagonal zone makes the relationship between dome and walls much easier to read.