The several pieces sawed from a log are not of the same quality, but vary more or less in grade. The slabs, covered on one side with bark, and the culls or very unsound pieces, are of no value as lumber. The ungraded lumber from a log is known as log run. In large mills, log run is graded as to quality after it is finally sent from the saw on roller conveyors and the several grades are stacked separately in such a way as to allow ready access of air for seasoning and to shed rain. After drying, and when removing it from the stacks for shipping, the lumber is further subject to an adopted form of inspection, in which each niece is graded for the purchaser's acceptance.