In mechanical engineering, a fit refers to the dimensional relationship of two parts that are to fit together without reworking. Fitting systems are used to manufacture one of the two workpieces that are to produce a fit as uniformly as possible and to move necessary tolerances into the other workpiece. The history of modern fitting systems dates back to the early twentieth century. Even before 1914, several companies had developed fitting systems, which were subsequently developed into the system of DIN fits after the First World War. Fits are therefore standardized. For fit specifications in technical drawings, internal dimensions and external dimensions are considered separately. The unit bore, along with the unit shaft, is the more widely used of two methods. According to these fit systems, different fits can result: clearance fits, i.e. fits in which a clearance is allowed between the two parts at the end; transition fits, which can be joined with rather low pressure; finally, interference fits, in which the two parts can be joined at the end only with pressure or high pressure.