175. Yet, here again we are met by another necessity for discrimination. There may be a true delight in the inlaying of white on dark, as there is a true delight in vigorous rounding. Nevertheless, the general law is always, that, the lighter the incisions, and the broader the surface, the grander, caeteris paribus, will be the work. Of the structural terms of that work you now know enough to understand that the schools of good sculpture, considered in relation to projection, divide themselves into four entirely distinct groups:-- 1st. Flat Relief, in which the surface is, in many places, absolutely flat; and the expression depends greatly on the lines of its outer contour, and on fine incisions within them. 2d. Round Relief, in which, as in the best coins, the sculptured mass projects so as to be capable of complete modulation into form, but is not anywhere undercut. The formation of a coin by the blow of a die necessitates, of course, the severest obedience to this law. 3d. Edged Relief. Undercutting admitted, so as to throw out the forms against a background of shadow. 4th. Full Relief. The statue completely solid in form, and unreduced in retreating depth of it, yet connected locally with some definite part of the building, so as to be still dependent on the shadow of its background and direction of protective line. 176. Let me recommend you at once to take what pains may be needful to enable you to distinguish these four kinds of sculpture, for the distinctions between them are not founded on mere differences in gradation of depth. They are truly four species, or orders, of sculpture, separated from each other by determined characters. I have used, you may have noted, hitherto in my Lectures, the word 'bas-relief' almost indiscriminately for all, because the degree of lowness or highness of relief is not the question, but the _method_ of relief. Observe again, therefore-- [Illustration: XII. BRANCH OF PHILLYREA.] A. If a portion of the surface is absolutely flat, you have the first order--Flat Relief. B. If every portion of the surface is rounded, but none undercut, you have Round Relief--essentially that of seals and coins. C. If any part of the edges be undercut, but the general protection of solid form reduced, you have what I think you may conveniently call Foliate Relief,--the parts of the design overlapping each other, in places, like edges of leaves. D. If the undercutting is bold and deep, and the projection of solid form unreduced, you have Full Relief. Learn these four names at once by heart:-- Flat Relief. Round Relief. Foliate Relief. Full Relief.