In GD&T, what does parallelism mean?

• A. The tolerance that a feature must be parallel to a referenced datum plane or axis.
• B. The tolerance that a feature must be coaxial with a datum axis.
• C. A measure of concentricity to a datum axis.
• D. The tolerance that a feature must be inclined relative to a datum.

Answer: (C)

Parallelism controls how a feature is oriented relative to a reference datum. It requires the surface or axis to run in the same direction as the referenced datum plane or datum axis within the specified tolerance. The tolerance zone is formed by two parallel planes (for a surface) or a cylindrical zone around an axis, oriented parallel to the datum. This keeps the feature from tilting away from true parallel orientation.

Concentricity or coaxiality, by contrast, cares about how centered the feature’s axis is about another axis, not about its direction. The idea of being inclined or angled to a datum describes a different tolerance (angularity), not parallelism.