Design Nomenclature / Systems & Spec
Datum
Also called datum feature, datum reference frame, DRF
A theoretically exact reference — plane, axis, or point — derived from a real feature and used to orient and locate everything measured from it. Datums are ordered primary, secondary, tertiary, and that order is the inspection setup: change it and identical tolerances measure differently.
Not to be confused with
- Basic dimensionIn GD&T, an exact theoretical value — boxed on the drawing — that locates or sizes a feature and carries no tolerance of its own. The tolerance lives in the feature control frame that refers to it, measured from the datums. ISO calls the same thing a theoretically exact dimension.
- Line conventionsThe graded weights and dash patterns that carry meaning on a drawing: thick continuous for visible edges, dashed for hidden ones, chain-dot for centrelines and axes, thin continuous for dimension and extension lines, and a leader arrowing a note to the feature it describes. Weight is information, not styling.
Origin
Latin datum, “a thing given”, the singular of data. The name records what it is: not a measurement, but the given from which measurements are taken.