In GD&T, a datum is a theoretically exact reference used to establish where a part is measured from. A datum may be a plane, an axis, or a point. The real surface, bore, or pin on the part used to establish it is called a datum feature.
Datum feature versus datum
A broad mounting face can be a datum feature, but it is not itself the perfect datum plane. During inspection, a surface plate, fixture, CMM, or other simulator contacts the real feature and establishes the theoretical reference from it. Keeping that distinction clear prevents many setup mistakes.
Primary, secondary, and tertiary datums
When a feature control frame lists A, B, then C, the order matters. Primary datum A establishes the first contact and removes the most movement. Secondary B and tertiary C complete a stable reference frame. The order should reflect how the part functions or assembles.
How to choose functional datums
The best datum features are usually the ones that locate the part in its real use, such as a mounting face, locating bore, or functional side surface. A convenient manufacturing surface is not automatically a good datum if it does not represent the part’s functional relationship.
Simple datum setup
A base face can establish datum A, a side face can establish B, and a bore can establish C. A hole pattern can then be located from that repeatable A-B-C setup. Explore the full GD&T datums guide.
Frequently asked questions
Datums in GD&T Explained: Datum Features and Datum Reference Frames FAQ
What is a datum in GD&T?
A datum is a theoretically exact plane, axis, or point used as a reference for locating or orienting a part during inspection.
What is a datum feature?
A datum feature is the physical feature on the part used to establish a datum, such as a surface, bore, pin, or slot.
Why are datums listed in order?
The order defines precedence in the datum reference frame and creates a repeatable inspection setup.
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