Plaster and lath
The older wall system of plaster troweled over thin wood strips, found before drywall.
Before drywall became standard in the mid-20th century, interior walls were built from plaster spread over lath — thin wood (or later metal) strips nailed across the studs. Plaster-and-lath walls are thicker, more soundproof, and more fire-resistant than drywall, but they crack as a house settles and the plaster can pull away from the lath ('keys' breaking), leaving sagging or hollow-sounding areas. Small cracks are normal and repairable; large bulging sections need re-anchoring or patching with setting-type compound rather than standard drywall mud. Anchoring heavy items requires hitting the studs or using plaster-rated anchors.