What is a datum feature and why is it important in GD&T?

• A. It is a decorative feature with no measurement role.
• B. It is a reference for measuring other features; it establishes the measurement base.
• C. It defines the surface finish requirements.
• D. It specifies the maximum material condition.

Answer: (B)

In GD&T, datums create the reference frame used to inspect a part. A datum feature is the actual surface, edge, or axis on the part that is chosen to establish one of those datums. This feature provides a real, measurable reference from which other features are located, oriented, or measured. In practice, inspectors align or gauge this feature to set the datum, and then they measure or verify other features relative to that datum frame. That’s why a datum feature is described as the reference that establishes the measurement base for the part.

This isn’t about decoration, surface finish requirements, or size limits like maximum material condition. Those serve different purposes in GD&T; the datum feature specifically defines the starting point for measurement and the coordinate system used in tolerance evaluation.