Parallelism is a GD&T orientation control that keeps a surface, axis or centre plane parallel to a datum. For a surface the zone is two planes parallel to the datum and the tolerance apart, so it limits tilt and flatness together but not the distance between the faces.
The classic use is the top of a block that must stay parallel to its base: datum A is the base and the top must fit between two planes parallel to A. The planes can move up and down to suit the part, so the thickness is controlled separately by its size tolerance.
Parallelism also applies to axes: a bore that must run parallel to a mounting face, or two shafts that must be parallel in a gearbox. With a diameter symbol the zone becomes a cylinder.
Because the zone is locked parallel to the datum, parallelism also limits flatness of the face. A separate flatness callout only adds something if it is tighter.
For a surface: two parallel planes, the tolerance value apart, parallel to the datum plane. For an axis with Ø: a cylinder parallel to the datum.
With Rule #1 in force, parallelism of a face to the opposite face is also limited by the thickness tolerance, so it is specified when a tighter value is needed.
Parallelism always needs a datum. It is usually a single primary datum, the surface the part sits on.
| Modifier | With parallelism |
|---|---|
| MMC or LMC | Allowed on the axis or centre plane of a feature of size. |
| Tangent plane (T) | Checks only the plane touching the high points of the face. |
| Diameter symbol | For an axis with a cylindrical zone. |
Example: a 25.0 ± 0.2 mm thick spacer with parallelism 0.08 mm of the top face to datum A
Result: Parallelism 0.06 ≤ 0.08 and the size is in tolerance, so the spacer passes both.
Every GD&T symbol page: GD&T symbols chart
Yes. It always references a datum, the feature the controlled surface or axis must be parallel to.
Flatness has no datum and lets the planes tilt. Parallelism locks them parallel to a datum, so it controls flatness and tilt together.
No. The two-plane zone can move closer or further from the datum. The distance is controlled by the size tolerance.
Yes, when it controls the axis or centre plane of a feature of size such as a bore or slot.
Rest the datum face on a surface plate and sweep an indicator across the controlled face; the total indicator movement is the parallelism.
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