1911 Encyclopædia Britannica/Rear Vault

REAR VAULT (Fr. arrière voussure), the term in architecture employed for the vault of the internal hood of a doorway or window to which a splay has been given on the reveal; sometimes the vaulting surface is terminated by a small rib known as the scoinson rib (q.v.), and a further development is given by angle shafts carrying this rib, known as scoinsoin shafts.