GD&T should express function. Datums establish the reference frame; geometric tolerances then control location, orientation, form or profile relative to that frame.
Why GD&T exists
Traditional plus/minus dimensions can describe size but may communicate functional relationships poorly. GD&T provides a structured language for controlling geometry and inspection.
Datum reference frame
A datum is a theoretically exact reference derived from a real feature. A primary, secondary and tertiary datum sequence constrains the part progressively so manufacturing and inspection share the same reference logic.
Common control families
- Form: straightness, flatness, circularity, cylindricity.
- Orientation: parallelism, perpendicularity, angularity.
- Location: position, concentric relationships in applicable standards.
- Profile: profile of a line/surface.
- Runout: circular and total runout.
Position tolerance
Position is widely used for hole patterns because it controls the allowable location error inside a tolerance zone relative to specified datums. It can be more functionally meaningful than independent ±X/±Y tolerances.
Do not add symbols without a measurement plan
If the shop cannot establish the datums or verify the tolerance, the drawing is not helping. The design, process and inspection method should be considered together.
This article is general guidance for preliminary engineering and shop-floor understanding. The drawing, applicable standard, machine/tooling OEM instructions, process-specific safety requirements and qualified engineering judgment take priority.