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Mass production relies on economical manufacturing methods, often involving complete part replacement (e.g., in aerospace). Dimensional limits, based on allowances and tolerances, ensure parts assemble easily. A basic size has plus/minus tolerances. Allowances create specific fits; for example, in a shaft-hole system, one part (shaft or hole) is made precisely to the nominal size, while the other incorporates the allowance for easier manufacturing. This can be 'shaft-based' or 'hole-based,' with hole-based being more common. The allowance is the difference between the shaft's high limit and the hole's low limit; both have high/low limits and tolerances.

Dimensions Mass production has long been the basis of the approach to the most economical manufacturing methods. The completa replacement of a defective item is common practice in the maintenance of aircraft and aerospace components For this reason, limits are imposed on the manufacturing processes to ensure that, if any two mating parts are manufactured to the dimensions as stated on the relevant drawings, then the parts assemble without the need of further major adjustments and in the least time possible The limits are based on the allowances and tolerances imposed on the dimensions of the manufactured parts. These dimensions are given the accuracy re quired by the designer of the respective parts.
The basic size of a dimension is given with tolerances expressed as a plus/minus range.
Allowances An allowance is a difference in a dimension necessary to give a particular 'class of fit' between two parts. If, for example (and using a typical limit system), a shaft was required to locate with a corresponding hole in a component. Then, to assist in the manufacture's economy, either the hole or the shaft is made as accurately as possible to the nominal size and a'1 allowance is applied to the associated item. The term shaft also includes bolts and pins. If the shaft is constant and the hole varies in size, then the systemnis 'shaft based'. If the hole is constant and the shaft varies in size, then the system is 'hole based'. The hole-based system is the one in more general use
The item dimensionedto include the allowance also has high and low limits and tolerance. The correct allowance would be the difference between the high limit of the shaft and the hole's low limit