Coordination refers to the relationshipbetween holes and shaft tolerance zones that have the same basic dimensions andare combined with each other. According to the relationship between thetolerance zones of the hole axis, the fit can be divided into three categories:clearance fit, interference fit, and transition fit. Fitting tolerance refersto the allowable variation in clearance or interference, which represents thefitting accuracy and is an important indicator for evaluating fitting quality.The fitting tolerances of the above three types of hole axis fittingrelationships are the algebraic sum of hole tolerance and axis tolerance.Therefore, the precision of the fit between the hole and the shaft depends onthe dimensional accuracy (dimensional tolerance) of the holes and shafts thatfit together. During design, the dimensional tolerances of holes and shafts canbe determined based on the fitting tolerances.

The benchmark system refers to a system inwhich one of the parts that cooperate with each other is used as a benchmark,and a standard tolerance zone is selected to change the position of thetolerance zone of another part (non benchmark), thereby forming variouscombinations. According to the different reference components, they are dividedinto base hole system and base shaft system. According to the differentrelative positions of the hole and shaft tolerance zones, both referencesystems can form clearances, interference fit, and transition fit.

The standard tolerance values are uniformlyspecified by national standards and divided into 20 tolerance levels withdifferent levels of dimensional accuracy that can be achieved by differentprocessing methods. IT01-IT18， The levels decrease sequentially, and the standard tolerance valuesincrease sequentially. Among them, the high-precision levels IT01-IT1 considerthe influence of measurement errors, and the standard tolerance is linearlyrelated to the basic dimensions. Different precision levels correspond todifferent standard tolerance calculation formulas, which are not only relatedto basic dimensions and tolerance level coefficients, but also involve factorssuch as machining errors and measurement errors of parts in different sizesegments. Especially, measurement errors caused by temperature changes canaffect the value of tolerance units.