¶ 15.49 Surface Shading Necessary
The drawing figures should be appropriately and adequately shaded to show clearly the character and/or contour of all surfaces represented. See 37 CFR 1.152. This is of particular importance in the showing of three (3) dimensional articles where it is necessary to delineate plane, concave, convex, raised, and/or depressed surfaces of the subject matter, and to distinguish between open and closed areas. Solid black surface shading is not permitted except when used to represent the color black as well as color contrast.