Until the Renaissance sculptors didn’t try to put backgrounds into their reliefs the way painters do. The simple perspective trick of making the more distant figures smaller than the up-front ones doesn’t work in a relief—that is, the viewer isn’t fooled into thinking that one of the figures is farther off: he merely sees one big figure and one baby figure up in the air above it.
The great reliefs on the frieze of the Parthenon, carved by Phidias in about 450 BC, are just front-line figures on a marble slab. The empty area all around them is supposed to be—the eye allows it—air, just air. There is no attempt at scenery or depth, except what you can easily achieve by setting two or more figures side-by-side, like these horses and their riders.
From the Parthenon frieze in the British Museum