Which of the following describes the relationship between datums and measurement methods?

• A. Any datum can be used for any measurement without constraints.
• B. In a properly defined system, datums determine the measurement frame and guide how measurements are evaluated.
• C. Measurement methods define the datums post-design.
• D. Datums are solely for documentation, not measurement.

Answer: (B)

Datums set up the reference frame for measurement. They establish where the primary, secondary, and tertiary datum features are located and oriented, which in turn defines how the part is positioned in space for inspection. Because the datum frame fixes location and orientation, all measurement operations and how tolerances are interpreted rely on that frame. In a properly defined system, the measurement method is built around these datums, guiding where probes or fixtures touch the part and how results are evaluated against the specified tolerances. This ensures measurements are repeatable and comparable across setups.

Understanding why the other ideas don’t fit helps reinforce the concept: datums aren’t arbitrary or interchangeable, and they constrain how you measure rather than being something you can just pick for convenience. They aren’t created after choosing a measurement method. And they aren’t just for documentation; they actively shape how measurements are performed and judged.