The dimensions of mating parts are generally controlled to have a proper fitting of matching parts for its optimal functional requirement. Providing dimensions on components or parts is the job of a product designer.
Interchangeability of the parts is, therefore, a major pre-requisite for economic production, operation and maintenance of machinery mechanism and instruments. It is therefore very much possible to interchange spare parts in various machines, tractors, motor cars, machines tools, airplanes and many, others so that they can be dismantled for replacement of parts in service conditions in the field, and also in many local workshops with least possible loss of time.
In order to produce interchangeable or identical parts, the components of interchangeable system and the various terms related with inter changeability of the mating parts should be understood clearly by the product designing, manufacturing and product inspecting staff working in industries.