At the time of assembling nested divisions, unless the dimensions are within controlled nest dimension tolerances, at the time of assembly, it is possible that some parts cannot be inserted into the holes, or a large gap to be present.
"Mating" is a concept of controlling the dimensional tolerances of such matching parts.

As is typified by the relationship between a shaft and its bearings, "mating" is a concept of controlling the permissible dimensional tolerances of the shape of the shaft and the shape of the hole, and the following three are the methods of mating.

(1) Gap mating

This is the method of mating in which there is always a gap in any combination of the shaft dimensional tolerance and the hole dimensional tolerance.

(2) Intermediate mating

This is the method of mating in which a gap may be generated or a tight mating margin may be generated depending on the combination of the shaft dimensional tolerance and the hole dimensional tolerance.

(3) Tight mating

This is the method of mating in which a tight mating margin is present at all times in any combination of the shaft dimensional tolerance and the hole dimensional tolerance.

In addition, there are the two methods 'hole reference method" and "shaft reference method" of machining the hole and shaft that mate with each other of taking either the hole or the shaft as the reference and making the other match with the reference.

In the hole reference method, the dimensional tolerances of the hole are set in advance as the reference, and the dimensional tolerances of the mating shaft are made to match with the reference dimensional tolerances. In this method it is possible to realize relatively easily the desired mating by finishing the dimensions of the shaft accurately.
The hole reference method is very frequently used for the mating of the cavity and core in the molds for plastic injection molding.

In the shaft reference method, the dimensional tolerances of the shaft are set in advance as the reference, and the dimensional tolerances of the mating hole are made to match with the reference dimensional tolerances. In this method, in order to machine the hole accurately to match with the shaft, a milling machine or a jig borer with a good accuracy will be required.

The mating method should be determined by the choice made by the designer of the mold. Which method is to be used for mating with which part has to be decided, and also it is necessary to decide what should be the permissible tolerances.