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In precision mechanics, fit refers to the degree of 'looseness' with which a shaft is inserted into a bored hole.
This coupling is related to the tolerance or allowance of both parts' dimensions. The shaft and the orifice must be of a similar diameter, otherwise there will not be a correct adjustment. With this in mind, measurements have been internationally standardised according to ISO regulation to ensure the interchangeability of items and their mass production.
Tolerance values are designated with a capital letter in the case of orifices and lower case letters in the case of shafts. The lower the value the higher the machining costs, as a greater precision is required.