Squinch

In architecture, a squinch is a structural element used to support the base of a circular or octagonal dome that surmounts a square-plan chamber.

Constructed from masonry, they have several forms, including: a graduated series of stepped arches; a hollow, open half-cone (like a funnel) laid horizontally; or a small half-dome niche.

[2] Squinches originated in the Sassanid Empire of Ancient Persia, remaining in use across Central and West Asia into modern times.

[5] An alternative approach to the structural problem of translating square space to round superstructure is the pendentive, much used in late Roman Empire and Byzantine architecture.

[4][7] The dome chamber in the Palace of Ardashir, the Sassanid king, in Firuzabad, Iran, is the earliest surviving example of the use of the squinch.