Flatness, straightness, and parallelism can all seem to describe a feature that should be “flat” or “straight,” but they control different things. The difference comes down to what feature is evaluated, whether the control uses a datum, and the shape of the tolerance zone.
Flatness controls one entire surface
Flatness is a form control. It keeps every point on the specified surface between two parallel planes. It does not use a datum because it does not compare the surface to another feature. Use it when a face itself must be flat for sealing, mounting, or contact.
Straightness controls a line or an axis
Surface straightness controls an individual line element on a surface. Axis straightness controls the derived axis of a feature of size and commonly uses a cylindrical zone. Neither type needs a datum because straightness is also a form control.
Parallelism compares a feature to a datum
Parallelism is an orientation control. It keeps a surface or axis parallel to a specified datum reference. A surface can be very flat but still not be parallel to the surface it must mate with, which is why flatness and parallelism are not interchangeable.
Quick choice
Use flatness when one mounting face must be uniformly flat. Use straightness when a rail, edge, or axis must not wander. Use parallelism when the feature must stay aligned to a datum.
Frequently asked questions
Flatness vs. Straightness vs. Parallelism: GD&T Differences Explained FAQ
Does flatness require a datum?
No. Flatness is a self-contained form control and does not reference a datum.
Can a surface be flat but not parallel?
Yes. A surface can be flat on its own but tilted relative to the datum surface it needs to align with.
Is straightness the same as flatness?
No. Straightness controls a line element or a feature axis; flatness controls an entire surface.
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