A relief is an even bigger fib than a statue; and the lower the relief, the harder it is to pull off the deceit.
A high relief is not simply fully-rounded figures stuck onto a flat marble slab. At some point—the place where those figures meet the slab and the rounding stops—the sculptor must lie. He must make it look as though his figure is not chopped off at the board and he must show the figure whole, though it isn’t.
And a low relief isn’t just a drawing on a flat board, with a few bulges in the middle corresponding to the most elevated parts, like hills, and the edges rounded down to the board. That won’t give the impression of rounded figures.
The only way to show the whole rounded figure—and that is the illusion you want to create—is to lie about its roundness already from the beginning. You must compress it, flatten it, make a kind of bellows out of it, with a lot of folds. These folds are the famous “planes” art explainers always talk about. They are the chief means of cheating the eye. The lower the relief, the more trickery there is to do, the more planes you need to pull it off.
Again, what is meant by “planes”? Planes are little steps on your flat slab: seen from the side, your relief doesn’t look so much like a big bump as like a step-pyramid. Each of these little, often extremely subtle, sometimes crooked, steps fools the eye into projecting depth onto what it sees. The eye has been educated all its life in doing this since in fact it sees flat (except for the slightly side-view we get from our second eye—the second point of view). A painter learns right away to represent the hundreds of little clues of receding lines, of light and dark, of color, of size, that tell the eye how to interpret what it sees—how to “read” it for depth. The sculptor has only a few of these clues to work with, and the most important is these planes.
Here are illustrations showing the planes a sculptor could indicate to give the impression of a rounded arm, leg, and foot seen from the front.
And a finished study of a nude in relief.