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Parallelism is the condition of a feature which is equidistant at all points from a datum. It can be thought of as zero degree of angularity.
When the parallelism of a surface is controlled relative to a datum plane, the tolerance zone is confined by two parallel planes which are parallel to the datum plane.The feature control frame contains the parallelism characteristic symbol, the tolerance and datum reference. It is attached to the controlled feature with a leader or extension line.

Parallelisum of a surface to a Datum plane.


Parallelism of an Axis to a datum plane with MMC Modifier

The following example, the MMC modifier is shown with the tolerance. The datum is a plane surface so it cannot be modified. In this case the width of the tolerance zone is dependent on the actual size of the feature. At MMC, the tolerance is 0.1 as indicated in the feature control frame. As the feature departs from MMC, the parallelism tolerance increases until at LMC least material condition.

parallelisum GD&T

Feature size
parallel item tolerance zone "X'



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