Relief sculpture is carved or modelled so that its forms project from a flat background surface, rather than standing entirely free and viewable from every angle the way sculpture in the round does. Because a relief remains physically attached to, and dependent on, its background, a relief sculptor works with genuine physical depth measured in inches rather than the full spatial freedom of a free-standing figure, and different reliefs use very different amounts of that available depth, from barely raised low relief to deeply undercut high relief that can approach the presence of a free-standing form.
Low relief keeps depth shallow and reads mostly from directly in front
Low relief, sometimes called bas-relief, projects only slightly from its background, with the difference in depth between the raised forms and the surrounding surface often measured in a fraction of an inch. This shallow depth means low relief tends to read most clearly when viewed roughly straight on, relying heavily on the play of light and shadow across its subtly raised surface, along with carved linear detail, to suggest volume and form that the actual physical depth alone couldn't fully convey. Much architectural and decorative relief carving, on building facades and coins alike, uses this shallow, low-relief approach specifically because it survives handling and weather well while still remaining clearly legible.
High relief pushes forms further out and can undercut them almost free
High relief projects considerably further from its background, often more than half of a form's full depth, and can include genuine undercutting, where part of a raised form is carved so that it's undermined or separated from the background behind it, approaching the physical presence of a free-standing figure while technically remaining a relief attached to its surface. This greater depth lets a sculptor render a subject with a fuller sense of real volume and can support a much wider range of convincing viewing angles than low relief allows, but it also demands considerably more structural care, since deeply undercut projecting forms are physically more fragile and prone to damage than a shallow, low-relief surface.
What we're still unsure about
The basic technical distinction between low and high relief, and how each affects a sculpture's structural durability and viewing behaviour, is well established, long-documented sculptural craft knowledge. What's more genuinely a matter of ongoing art-historical debate, for many specific ancient and historical relief works, is exactly how the original artist intended a given relief to be viewed and lit, since surviving relief sculpture is often now displayed in museum lighting conditions quite different from its original architectural setting — art historians continue to debate how much a relief's now-altered viewing conditions change a modern viewer's experience of it relative to what its original audience actually saw.
This sits inside Relief Sculpture, one of seven topics in Sculpture, one of five domains in Art, one of seventeen subjects the app can quiz you on.