GD&T Symbol Guide
Datum
Establishes the theoretically exact point, axis, line, or plane used as the anchor for measuring related GD&T controls.
Definition
A datum is theoretically exact. A datum feature is the real surface or feature of size on the part that is used to establish that datum.
Think of datums as the measurement anchors for the part. They remove degrees of freedom so related features can be evaluated consistently.
Application
Use datum features on functional faces, mounting surfaces, bores, slots, or tabs that control how the part fits or is inspected.
The datum letter appears again in feature control frames to show which reference frame the tolerance zone is built from.
Datum Feature A Establishing A Measurement Plane
The boxed datum letter identifies the physical feature used to simulate the datum during inspection.
3D Tolerance Zone
A datum does not create a tolerance zone by itself; it creates the stable reference frame used by other zones.
Inspection Method
A datum plane may be simulated by a surface plate, fixture, CMM plane, or other inspection equipment.
For a feature of size, the datum is usually the derived axis or center plane rather than the physical surface itself.
Worked Check: Choosing A Stable Datum
A bracket mounts to a machine base on one large face, then two holes locate a cover. The mounting face is labeled datum A.
Functional contact
Large face = A
The mounting face is the first surface that stabilizes the part in assembly.
Reference frame
A then B/C
A establishes orientation before hole locations are checked from secondary datums.
Inspection setup
Simulate A
The part is seated against a flat inspection surface or CMM plane before related checks.
Datum A gives every downstream measurement a repeatable starting point, instead of measuring the holes from an arbitrary edge.
Comparison Table
| Term | Physical? | Used For | Example |
|---|---|---|---|
| Datum | No | The exact reference | A perfect plane or axis |
| Datum feature | Yes | The real part feature | Mounting face or bore |
| Datum reference frame | No | Measurement coordinate system | A|B|C in a callout |
Notes
Datum feature letters are shown in boxes and attached to the controlled feature with a triangle, leader, extension line, or related feature control frame.
Form controls such as flatness, straightness, circularity, and cylindricity do not use datums because they control shape only.