Level Up Your Skills with the Technician Development Program I Exam 2026 – Ready, Set, Succeed!

The reference frame for measurements in GD&T is established by a datum. A datum is a theoretically exact point, line, or plane derived from a feature of size or surface, used to define the origin and orientation for all measurements. By assigning primary, secondary, and tertiary datums, you create a datum reference frame that fixes how the part is located and oriented during inspection, so tolerance zones are interpreted consistently regardless of how the part was manufactured or measured.

Limits specify the allowable size range, not the measurement frame. Tolerance defines how much variation is allowed, but needs a frame to be meaningful. A feature control frame communicates the tolerance application, not the reference frame itself.