What is a squinch?

A squinch is a small supporting arch or vault built across the upper corner of a square room, bridging the corner so that an octagonal or round base is formed on which a dome can rest. It often takes the shape of a niche or half-cone opening forwards; sometimes it is a simple arch across the corner, or a series of masonry courses corbelled out layer by layer. In Dutch it is called a tromp, in German a Trompe.

How it works

A dome is round, a room is usually square. A squinch is placed in each of the four corners. That cuts off the corners and turns the square into an octagon. On that octagon you can build an eight-sided dome, or add a further transition to a circle for a round dome or a dome drum.

Squinch versus pendentive

Both solve the same problem in different ways:

• A squinch bridges the corner with a niche or arch and produces an octagon.
• A pendentive is a curved triangular surface, part of a sphere, that rises to a complete circle. It gives a smoother transition and suits larger domes.

Uses and examples

• Persian architecture: the Palace of Ardashir near Firuzabad (Iran) is regarded as the earliest surviving example of squinches under a dome.
• Romanesque churches: crossing domes on squinches, as at Notre-Dame-du-Port in Clermont-Ferrand.
• Byzantine and Armenian churches, especially for domes of smaller diameter.
• Islamic architecture, where squinches are often filled with muqarnas (stalactite vaulting).

Practical tips

• Spotting one: look into the corners below a dome. A niche or arch forming an octagon is a squinch; a smooth, curved triangle is a pendentive.
• Cracks: cracking in squinches can indicate movement in the dome or walls. Have it assessed by a structural engineer experienced with historic masonry.

Related terms

• Pendentive
• Dome
• Dome drum
• Muqarnas
• Corbel
• Corbelled vault

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