GROIN
, with Builders, is the angle made by the intersection of two arches. It is of two kinds, regular and irregular; viz, Regular when both the arches have the same diameter, but an Irregular Groin when one arch is a semicircle and the other a semiellipsis. Groins are chiefly used in forming arched roofs, where one hollow arched vault intersects with another; as in the roofs of most churches, and some cellars in large houses.