Ashlar forms the main feature in true masonry. The stones are always set in true courses, and the depth may be from 12 in. to any available thickness. The beds and joints should always be chisel dressed; that is, drafted and boasted off. The stones for the facing of the wall are generally 2 ft. -1 in. to 2 ft. 6 in. long, 12-18 in. deep, and 4 1/2-10 in. thick, headers being thickest. It is a common practice to cut the stones in a somewhat tapering form, so that they closely abut only for a short distance (1-4 in.) back from the face, the spaces thus left being filled in with mortar and chips. The joints are either left close, or dressed back with a square or triangular recess. Hewn ashlar masonry set stone and stone, or with thin beds of mortar, and having the face-work backed up with rubble or bricks, is always weak, and will not preserve a true line on the face either vertically or horizontally, owing to unequal shrinkage.