An International Tolerance grade number setting the width of a tolerance zone.
IT grades run IT01 to IT18, with lower numbers tighter. The value depends on the grade and the nominal size: IT7 is 10 µm at 3 mm and 30 µm at 80 mm.
The grade describes the tolerance width only. Where that zone sits relative to nominal size is set by the deviation letter — so 30h7 and 30g7 have the same width of 21 µm but sit in different places.
Reading an IT grade as an absolute tolerance. It is a grade number; the micrometre value must be looked up against the nominal size.
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