description of selective assembly that aptly summarizes the concept. One might ask, if parts must be selected for mating, then what makes selective assembly any different from the oldest craft methods? But there is in fact a significant difference. Selective assembly merely grades the parts into several ranges; within each range, there is still random interchangeability. This is quite different from the older method of fitting by a craftsman, where each mated set of parts is specifically filed to fit each part with a specific, unique counterpart. Random assembly not available: oversize and undersize parts In contexts where the application requires extremely tight (narrow) tolerance ranges, the requirement may push slightly past the limit of the ability of the machining and other processes (stamping, rolling, bending, etc.) to stay within the range. In such cases, selective assembly is used to compensate for a lack of total interchangeability among the parts. Thus, for a pin that must have a sliding fit in its hole (free but not sloppy), the dimension may be spec'd as 12.00 +0 −0.01 mm for the pin, and 12.00 +.01 −0 for the hole. Pins that came out oversize (say a pin at 12.003mm diameter) are not necessarily scrap, but they can only be mated with counterparts that also came out oversize (say a hole at 12.013mm). The same is then true for matching undersize parts with undersize counterparts. Inherent in this example is that for this product's application, the 12 mm dimension does not require extreme accuracy, but the desired fit between the parts does require good precision (see the article on accuracy and precision). This allows the makers to "cheat a little" on total interchangeability in order to get more value out of the manufacturing effort by reducing the rejection rate (scrap rate). This is a sound engineering decision as long as the application and context support it. For example, for machines for which there is no intention for any future field service of a parts-replacing nature (but rather only simple replacement of the whole unit), this makes good economic sense. It lowers the unit cost of the products, and it does not impede future service work. An example of a product that might benefit from this approach could be a car transmission where there is no expectation that the field service person will repair the old transmission; instead, he will simply swap in a