Ashlar is finely dressed (cut, worked) masonry, either an individual stone that has been worked until squared or the masonry built of such stone. It is the finest stone masonry unit, generally cuboid, mentioned by Vitruvius as opus isodomum, or less frequently trapezoidal. Precisely cut “on all faces adjacent to those of other stones”, ashlar is capable of very thin joints between blocks, and the visible face of the stone may be as quarry-faced or feature a variety of treatments: tooled, smoothly polished or rendered with another material for decorative effect.