Individual pieces of timber, as they come off the saw, have widely different strength properties; it is not uncommon for some pieces to be four or more times stronger than others of the same size and species. If all sawn timber were to be used without selection for structural work as joists, rafters, etc., it would be necessary to assume strength values appropriate to the weakest pieces. This would not be very efficient and obviously it would be better to separate the pieces into groups or grades, so that higher strength values could be assigned to the better material. This classification, on the basis of expected strength, is known as stress grading.