Kenneth G. Wilson (1923–). The Columbia Guide to Standard American English. 1993.
concave, convex (adjs.)
The circumference, or outer edge, of a concave object is closer to the viewer, and its center curves farther away; convex objects bulge the opposite way, with the center close to the viewer and the edge or circumference curved farther away. One-half of a hollow rubber ball illustrates both forms: viewed from inside, it’s concave; from the outside, it’s convex. (Mnemonic: a cave—as in concave—is hollow too.)
