metric tolerance

let see an example: the diameter of a shaft is 16h6

16 means, the nominal diameter of the shaft is 16mm (basic size)

h means, where is the tolerance field locating compare to the nominal size, in this case the shaft could be smaller or equal than 16 mm

6 means, the size of the tolerance zone from the chart or the international tolerance grade (IT number), which is in this case 0 to -0.011mm

the dimension of this shaft could be: min: 15.989 mm, max: 16.00 mm

see ISO tolerance chart: http://www.fullermetric.com/technical/information/tech_iso_tolerances.htm