There are several causes of workpiece variation, including machine inaccuracies, tool wear, material variations, and human factors. Terms used in dimensioning workpieces include nominal size, basic size, allowance, tolerance, and limit. Tolerances allow for permissible variation from the basic size. Tolerance stacking is the accumulation of tolerances that can cause a part to be out of specification. Methods to control tolerance stacking include baseline dimensioning and selecting processing sequences that do not require tolerance tightening. Proper tolerance selection is important to control costs and maintain dimensional accuracy.