How do thousands of tiny niches turn a square room into a dome?

Muqarnas turn a square room into a dome by stacking many small niche-like cells, tier on tier, each projecting over the ones below like a corbel, until a smooth, decorative zone of transition leads the eye from the square walls to the round dome. It is an invention of Islamic architecture, and its structure originated from the squinch, a corner support that carries a dome over a square room. Its main function is ornamental: it is typically used to obscure or fill the structural transitions in a building.

The cells are not random. Each is one of a limited set of simple prismatic shapes, combined according to precise rules, and the result can be highly complex. From below, the carved surfaces create an effect of light and shadow, and the cells are sometimes referred to as honeycombs or stalactites.

Muqarnas most likely first developed in eleventh-century Iraq, though the earliest preserved examples appear outside it too, and scholars still debate whether it appeared at several places at once or spread from one. The material changes with the place: stone in Syria, Egypt and Turkey, plaster or wood in North Africa, brick in Iran and Iraq.

The most dazzling example is in the Alhambra. The dome of the Hall of the Two Sisters has at least 5,000 cells that unfold from a central summit down into sixteen miniature domes. What did it mean? Oleg Grabar suggested that such domes represented the rotating heavens, while Yasser Tabbaa saw an atomist view of the universe, a dome divided into small units held together by God.