In geometric dimensioning and tolerancing, a datum feature serves as what?

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

Answer: (C)

A datum feature serves as the reference surface, axis, or point used to establish the datum reference frame for measuring the part. This creates a stable measurement base so that all other features are measured consistently relative to that defined origin and orientation. By designating a primary datum feature (the main reference), then secondary and tertiary datums to further constrain the part, inspectors can align the part in a fixture or on a CMM and check tolerances in a meaningful, repeatable way. For example, a flat machined surface might be chosen as the primary datum because it defines the part’s orientation and location for locating holes or features relative to that plane. This foundational role—providing a reference for measurement—is what a datum feature does, unlike decorative aspects, surface finish requirements, or other concepts that don’t establish the measurement base.