The moldings are the basic building blocks of classical architecture. They are divided into the following geometrical categories: plane, concave, convex, and compound.
Among the plane moldings, a small plane surface is referred to as a fascia; a very small plane surface, a fillet; and if it is recessed, it is referred to as a sunk fillet. Occasionally, one finds splayed or inclined plane surfaces.
The concave and convex moldings are typically based on sections of a circle: quarter circle, half circle, and three quarter circle, though they may be elliptical. A scotia can be a pure concave half circle but in the case of the attic base one finds it to be made up of two sections of circles with different radii. The thumb molding, which is a drooping torus, and Greek echinus also manifest a more complex geometry beyond the simple circle. These moldings can be made of part of a parabola or ellipsis. The quirk, or turning in, molding before a plane surface is often omitted in the Greek echinus.
Compound moldings combine convex and concave shapes. If one draws a wave along an horizontal axis and then takes a section of the wave containing the convex and concave shapes, the resulting profile would be a cyma recta. In fact, the term cyma recta means upright wave in Greek. To derive a cyma reversa, one would perform the same exercise with a wave along a vertical axis. A compound molding where the convex and concave shapes come together at angle is known as a beak molding.