SQUINCH, in architecture, a general term for several means by which a square or polygonal room has its upper corners filled in to form a support for a dome : by corbelling out the courses of masonry, each course projecting slightly beyond the one below; by building one or more arches diagonally across the corner; by building in the corner a niche with a half dome at its head, or by filling the corner with a little conical vault which has an arch on its outer diagonal face and its apex in the corner. The arched squinch seems to have been developed almost simulta neously by the Roman builders of the late imperial period and the Sassanians in Persia.
Squinches are often found both in Romanesque and Moham medan work. In Italy the form is either the conical type as in the church of S. Ambrogio at Milan (crossing 11th or 12th cen tury) or as a succession of arched rings as in the 13th century central tower of the abbey church at Chiaravalle Milanese; more complex forms, with niches and colonnettes are characteristic of the French Romanesque of Auvergne, as in the cathedral of Le Puy en Velay (late 11th and early 12th century); the allied churches of the south-west coast, such as St. Hilaire at Poitiers,
use conical squinches of the Italian type. Mohammedan archi tecture, borrowing from the Sassanian precedent, makes great use of squinch forms, particularly in the Syrian, Egyptian and Moor ish phases; the stalactite work, which is so marked a feature of later Muslim architecture, is, in essence, merely a decorative development of a combination of niche squinch forms. In Gothic architecture squinch arches are frequently used on the insides of square towers to support octagonal spires. (See BYZANTINE AND ROMANESQUE ARCHITECTURE; DOME ; MOHAMMEDAN ARCHITEC TURE ; PENDENTIVE.)