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Before discussing the specific objectives of this chapter, a short discussion is appropriate as to the meaning of and need for tolerances. No two things are made alike. This is just as true in industry as it is in nature. However, many of the differences are relatively greater and more easily discernable in nature than in industry. Regardless of this fact, failure to detect a difference is not to be construed as the absence of a difference. It is attributable, instead, to an inadequate sensitivity (or resolution) or precision of the detection system being used. As measurement precision increases, the ability to recognize existing differences increases. If this logic is acceptable, then it follows that a product design characteristic cannot be reproduced identically in its manufacture and that some degree of variation must be allowed in the manufacturing process. This allowable degree of variation is referred to as tolerance. Tolerancing, simply, is the act of specifying a tolerance. The most common sources of manufacturing variations are differences between and within raw materials, manufacturing methods, machines and tools, environments, and operators.