This section is from the book "The Building Trades Pocketbook", by International Correspondence Schools. Also available from Amazon: Building Trades Pocketbook: a Handy Manual of reference on Building Construction.
While the Greeks relied for effect on the graceful contour of their moldings, the Romans counted more upon the richness of carved ornament. Delicacy of execution in the Greek workmanship gave place to the mechanical and ostentatious in the decoration of Roman moldings. Besides this, the execution of Roman moldings was often very careless. As a general rule, the lines of enrichment or caring on both Greek and Roman moldings corresponded to the profile of the surface on which it was caned.
The torus, shown at (a), Fig. 5, is semicircular, the center being at c, the middle point of the line a b.
The cavetto or cove, shown at (b), is a concave molding whose profile is a quarter circle. The center a, is found by ing the lines d c and b a until they intersect.
The ovolo, echinus, or quarter-round. shown at (c), is a convex molding with a quarter-circle profile, the center d being found as shown.
The cyma recta, shown at (d), is made up of two quarter circles tangent at e. The centers f and g are found by bisect-ing the lines ca and b d.
The cyma reversa, shown at (e), is the reverse of the cyma recta, which is concave above and convex below, while the former is convex above and concave below. The drawing ex plains itself.
The scotia, shown at (f), is drawn as follows: Having given the points c and d, draw cd, and bisect cd at e. With center and ec as a radius draw a semicircle dfc. Draw dg an angle of 80° with the base fillet. cutting the arc at g. Bred a perpendicular at d. and with g as a center and a radius gd cut the perpendicular at h. Draw gh and drop a perpendicular from c, cutting gh at b. With b as a center and bc as a radius, draw the arc cjg. With h as a center draw the arc gid, completing the curve.