An architectural rib is a structural element used in the construction of arches and vaults. It is composed of four parts: a lierne, an ionic, an OGEE, and an OGIVE. A lierne is a curved rib that connects two other ribs at an angle. An ionic is a type of rib that has a convex curve at the top and a concave curve at the bottom. An OGEE is a double-curved rib with a pointed arch at the top and a curved arch at the bottom. Finally, an OGIVE is a pointed arch rib that is usually used to form the apex of a vault.