What is parallelism in GD&T?

• A. The orientation of a surface relative to a datum
• B. The perpendicular relationship of two features
• C. The angular relationship between two lines
• D. The alignment of two surfaces to stay equidistant along a direction

Answer: (D)

Parallelism in GD&T controls how well a surface or axis is kept parallel to a referenced datum direction. The tolerance zone is defined by two parallel planes (or a cylindrical zone around a datum axis) separated by the specified tolerance, and the feature must lie entirely within that zone. This setup ensures the feature remains at essentially the same distance from the datum as you move along the specified direction, keeping it equidistant.

That’s why the description about the alignment of two surfaces to stay equidistant along a direction is the best fit. It captures the idea of maintaining a constant separation relative to the datum as you move along the reference direction.

The other choices describe different concepts: orientation relative to a datum is too broad, perpendicularity focuses on right angles, and the angular relationship between lines concerns angles rather than consistent distance from a datum.