The movement of the runner on top of the bedstone creates a 'scissoring' action that grinds grain trapped between the stones.
Millstones come in pairs: a stationary base with a convex rim known as the bedstone (or nether millstone) and a concave-rimmed runner stone that rotates. They are sometimes referred to as grindstones or grinding stones. Millstones or mill stones are stones used in gristmills, used for triturating, crushing or, more specifically, grinding wheat or other grains. This is a runner stone a bedstone would not have the 'Spanish Cross' into which the supporting millrind fits.
For roadside distance markers, see Milestone. This article is about stones used to grind grains.
