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Hole-basis system (IT tolerances)

To avoid having to make holes in every possible size, the hole-basis system is used: the hole always gets a lower deviation of zero (letter H), and the type of fit is instead controlled by the shaft's tolerance zone. The IT grade (e.g. IT7) states how large the tolerance span itself is.

H hole: lower deviation=0\text{H hole: lower deviation} = 0definition of the hole-basis system

Symbols

HHtolerance zone with zero lower deviation
ITITtolerance grade (span)

Example

An H7 hole with nominal size 30 mm:

the smallest size is exactly 30.000 mm, the largest is set by IT7.

Only the hole is "H"; it is the shaft's letter and number that decide clearance or interference.
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Part of Machine Design: Tolerances and fits.