Large circular slab of coarse rock up to 1m in diameter and typically 0.2-0.3m thick used for grinding grain in a mill. One face is roughened by means of a pattern of lines cut into the surface while the other face may be slightly domed. Millstones were used in pairs (an upper and lower stone), a central hole in each taking the spindle that keeps them concentric and in the case of the upper stone attaches to the power source that turns it.