GROIN, in architecture, the edges formed at the intersections of two vaults at an angle to each other. If the vaults intersect at right angles and are of the same height and radius, these intersec tions will all lie on a vertical plane at 45° to the planes of the two vaults. If, however, one vault is lower than the other, or the curvatures are different, winding and distorted curves will result. Both the Roman and Renaissance constructors of vaults frequently regularized the rib shapes and slightly warped the surfaces of the vaults until they met at this regularized groin line. Thus in the coved, penetrated ceilings of the 16th century, where an elliptical vault was intersected by smaller vaults besides, the groins would often be given simple segmental curves, meeting at a point in the centre of each penetration, and the smaller vaults forming the penetrations given a surface almost conical in order to meet this line. The term Welsh groin is often applied to the groin resulting from such an intersection of a smaller cross vault with a higher maid vault. In the mediaeval period, when ribbed vaulting became common, the ribs under the groins (groin ribs or diagonal ribs), being built first, were usually on curves lying in a simple plane. The web, or filling in of small stones between the ribs, could be warped or twisted at will, so as to start correctly from the wall or cross ribs of different sizes and yet meet over the groin rib. (See VAULT.)