Parallelism
The orientation control that also does the job of flatness — which is why writing both on one face is usually one callout too many.
What it controls
Parallelism limits how far a surface or a derived axis may lean away from a datum. Zero degrees is implied, so no angle is written: the datum itself states the direction the feature is meant to follow.
Because the zone is fixed in angle to the datum but free to float, parallelism controls form as well as orientation. A surface that fits inside a parallelism zone of 0.05 mm is inside a flatness zone of 0.05 mm at the same time.
The tolerance zone
Two parallel planes a set distance apart, held parallel to the datum — or, when the value carries a diameter symbol and the frame sits on the size dimension, a cylinder of that diameter parallel to the datum, containing the derived axis. The feature has to lie inside, wherever the zone ends up floating.
Try it in 3D
Interactive 3D needs WebGL. The tolerance zone is described in full above.
Within tolerance
Drag to exaggerate the deviation. The part is shown far out of scale so the zone stays visible.
Reading the callout
This surface has to lie between two planes 0.05 mm apart that are parallel to datum A.
When to reach for it
- A mating face that has to sit flush against a base already used as a datum.
- A guide or slideway where an even gap matters more than the exact size of it.
- The top face of a plate that carries a fixture, where the work has to end up parallel to the machine table.
How it is measured
- The part rests on the datum surface and an indicator is swept across the toleranced face; the total swing is the parallelism.
- A CMM probes the datum feature first, fits the reference plane, then reports the toleranced surface in that frame.
- For a derived axis, the datum axis is established from the datum feature and the toleranced axis is compared with it rather than with the surface.
Getting it right
- Use the size dimension to control the distance between the faces, since parallelism fixes the angle and leaves the spacing to the dimension.
- Leave the flatness callout off a face that already carries parallelism at the same value — the parallelism zone already limits it.
- State which face is the datum, because parallelism between two surfaces with neither named is ambiguous in both directions.
Common questions
Does parallelism replace flatness?
At the same value, yes. A surface inside a parallelism zone of 0.05 mm is necessarily inside a flatness zone of 0.05 mm, because the two planes bounding the zone are parallel to each other as well as to the datum. Writing flatness tighter than parallelism is meaningful; writing them equal makes the flatness callout redundant.
Does parallelism control how far apart the faces are?
No, and this is the most common misreading. The tolerance zone floats: it can sit anywhere between the faces as long as the surface fits inside it. How far apart the two faces are is controlled by the linear dimension and its size tolerance, working alongside the parallelism rather than being replaced by it.
Why is no angle written in the callout?
Because zero is implied. Parallelism is the special case of angularity at zero degrees, so the standard leaves the angle out. If the feature is meant to sit at any other angle, angularity is the control and the angle has to be given as a theoretically exact dimension.
How is parallelism measured on the shop floor?
The datum surface is set down on a surface plate, and an indicator is run across the toleranced face. The difference between the highest and lowest readings is the parallelism, with no arithmetic needed. It is one of the few geometric tolerances that can be checked properly with a surface plate and an indicator alone, which is part of why it is so widely used.
These pages explain the published standards for reference and describe the tolerance zones they define. The values shown in the examples are illustrative. What a given part can hold depends on its geometry, material and process, and is agreed per drawing.