Today, we use metals fabricated at the mill, stone cut and dressed at the quarry, and concrete cast before erection. These prefabricated components are not easily customized on the job and must be fit to site-built frames. Suddenly construction tolerances become very important in ensuring that the puzzle pieces fit together with reasonable accuracy--puzzle pieces that may come from a dozen different manufacturers in a half-dozen competing industries. Allowances for the variation in sizes of clay brick are covered in ASTM C 216 for face brick, ASTM C 62 for building brick, and ASTM C 126 for glazed clay facing tile and facing brick. Face brick allowances are divided into Types FBX and FBS. The units are of equal quality, but Type FBX brick are held to tighter size tolerances. Type FBA brick are not governed by size tolerances. These units are meant to vary significantly in size to look like rough, hand-molded brick. Little is exact in the manufacturing, fabrication, and construction of buildings and building components. Tolerances allow for the fit and misfit of the various parts as they come together in the field, yet ensure structural safety, joint performance, secure anchorage, moisture resistance, and acceptable appearance.