How are datums chosen and ordered in a frame?

• A. Based on functional requirements; primary datum (A) first, then secondary (B), then tertiary (C).
• B. Alphabetical order of datum symbols.
• C. Size order from largest to smallest.
• D. Random order.

Answer: (A)

Datums are chosen to create a repeatable reference frame that aligns the part with how it will function and mate with other parts. The primary datum fixes the most influential location and orientation, serving as the main anchor for the part’s coordinate system. The secondary datum then constrains the remaining orientation or location, and the tertiary datum completes the frame by eliminating any remaining degree of freedom. The order is driven by functional requirements, not by size, alphabetical order, or randomness. In practice, you pick a datum feature that is stable, reproducible, and directly related to how the part interfaces with mating parts; the primary is typically a flat or base feature, the secondary an axis or perpendicular surface, and the tertiary a third feature to finish locking the frame. This setup ensures a consistent, inspectable reference for all tolerances.