Parallelism controls how far a surface or an axis may deviate from being parallel to a datum. Because it is an orientation control, it always takes a datum reference, and because the tolerance zone is two parallel planes at a fixed orientation, it also bounds the form of the feature within the same value. That second consequence is what makes a separate flatness callout usually redundant.
Surface parallelism
The controlled surface must lie between two planes separated by the tolerance value and parallel to the datum. Because those planes are parallel to each other, the surface's own flatness is also bounded by the same number. So a face with a parallelism callout does not additionally need a flatness callout unless the flatness has to be tighter than the parallelism, which is unusual.
Axis parallelism
Applied to a hole or a shaft, the zone is usually a cylinder parallel to the datum axis, within which the feature's derived axis must lie. This is how a bore is held parallel to a mounting face or to another bore, and it is the control that matters on anything with two shafts that must run true to each other.
How a shop holds it
By machining the controlled feature and the datum in the same setup wherever possible, because every re fixturing introduces error between them. Where two setups are unavoidable, the datum must be located precisely in the second one. On tight values, grinding on a surface grinder with the datum face on the magnetic chuck is the usual production route.
Common mistakes
Calling out parallelism and flatness on the same face with the same or a looser value, which pays for two inspections and controls one thing. Naming a datum the assembly does not actually locate from. And applying parallelism to a face whose real requirement is a distance, where a simple dimensional tolerance would have been both cheaper and clearer.
Questions people ask about parallelism gd&t
Does parallelism need a datum?
Always. It is an orientation control and is meaningless without something to be parallel to.
Does parallelism control flatness too?
Yes. The tolerance zone is two parallel planes, so the surface's flatness is bounded by the same value. A separate looser flatness callout is redundant.
How is parallelism measured?
By locating the part on its datum and measuring the controlled feature across its extent, usually on a coordinate measuring machine or with an indicator on a surface plate.
Parallelism or a dimensional tolerance?
If the requirement is really about distance, a dimensional tolerance is simpler and cheaper. Use parallelism where the orientation itself is what matters.