How did domes and squinches support the roofs of Sasanian buildings?

In Sasanian architecture, the transition from a square or polygonal room to a circular dome was achieved through the use of squinches. A squinch is a structural device that fills in the upper corners of a room to provide a stable base for the dome.1

Sasanian builders, alongside Roman architects of the late imperial period, were among the first to develop the arched squinch. These supports functioned by creating a structural transition that allowed the weight of the dome to be distributed across the corners of the square space. By bridging these corners, the squinch effectively transformed the square plan into an octagonal opening, which could then support the circular foundation of the dome. This technique was fundamental to Sasanian construction and later became a hallmark of Islamic architecture, where it was further developed into complex forms like the muqarnas.12