Before two components are assembled together,the relationship between the dimensions of the mating surfaces must be specified.In other words,the location of the zero line to which deviations are referred must be established for each of the two mating surfaces.This actually determines the degree of tightness or freedom for relative motion between the mating surfaces.There are basically three types of fits,namely,clearance fit,transition fit,and interference fit.In all cases of clearance fit,the upper limit of the shaft is always smaller than the lower limit of the mating hole.This is not the case in interference fit,where the lower limit of the shaft is always larger than the upper limit of the hole.The transition fit,as the name suggests,is an intermediate fit.According to ISO,the internal enveloped part is always referred to as the �shaft,�whereas the surrounding surface is referred to as the �hole.�Accordingly,from the fits point of view,a key is referred to as the shaft and the keyway as the hole.