leveller

Anything intended to bring about a level – i.e. a horizontal – surface; any contrivance by which the upper surface of a structure may be brought to a horizontal or nearly horizontal plane. Especially in stone masonry, a stone of the proper dimension to make up a difference in height between two adjoining stones, so as to produce a level bed for the next course above; also a flat stone for making a level bearing, as in a footing course. (Sturgis, 1900)