An undermount sink is mounted from underneath the countertop. The bowl hangs from the underside of the top, creating an uninterrupted line from surface into basin. There is no visible rim sitting on top of the counter, as there is with a drop-in basin. The point is visual continuity and a cleaner sightline. What varies is the reveal—the gap between the edge of the cut-out and the rim of the bowl. A positive reveal is a gap you can see; the cut-out is slightly larger than the bowl. A negative reveal is the opposite; the bowl rim sits slightly proud of the surface. A flush reveal has bowl rim and surface at the same height.
Each reveal reads differently. A positive reveal creates a shadow line, visual separation between counter and bowl. A negative reveal appears to have no gap at all from normal viewing height; the bowl reads as sitting in the surface. A flush reveal is exact: bowl rim and surface at the same level, no lip in either direction. Each reveal collects differently; what sits in the shadow of a positive reveal is hidden, while what collects on a flush or negative reveal is visible.
The cut-out edge in an undermount basin is a visible, finished surface. It is not hidden under a rim. The edge of the hole in the countertop is touched and seen every day. It has to be a finished edge—sanded smooth on stone, polished on granite, sealed on porous material. This is the detail that decides whether an undermount looks intentional or unfinished.
The mount itself places load on the countertop material. The bowl is supported and fixed from underneath, and that load concentrates at the attachment points. Some materials are marginal for undermounting—thin engineered surfaces can flex, brittle materials can fail at the stress concentration. This is why material choice and undermount mounting are not entirely separate decisions. A material suitable for undermounting is one that can take that load without concern.