A|B|C Datum precedence order
Datum precedence order · Datums y referencias · ISO 1101 + ASME Y14.5
Example in plan
How to interpret it
The A|B|C order is not arbitrary: plane A is fixed first, then the direction with B and finally the side location with C. Changing the order changes the interpretation.
What controls
The order in which datums are cited in the feature control frame (first A, then B, then C) is not a list of equivalent references: it defines the sequence in which the part is constrained. The primary seats it and fixes the degrees of freedom it can, the secondary orients what remains and the tertiary locks the last one. Changing the order physically changes how the part is supported and therefore the measured value: |A|B|C| and |B|A|C| are different requirements even though they cite the same features.
How to verify
Physically establish precedence in that same order: seat the primary datum on the surface plate or angle plate with stable contact, bring the part against the secondary stop and finally against the tertiary one; only then measure. On a CMM, program the alignment with the same hierarchy and check in the report that the software applied it that way: a good share of inspection errors come from an alignment done by overall best fit instead of by datum precedence.
Datums and notes
Each feature control frame has its own precedence: the same part can carry one frame |A|B|C| and another |B|A| without contradiction, because they control different functions. The primary datum is always the first one cited in that particular frame, even if its letter is B or C.
Workshop guideline values
- General
- Classic 3-2-1 arrangement (primary plane, secondary face and tertiary stop) with datums in the range of 0.05–0.2 mm.
- Accuracy
- Datums prepared to 0.01–0.03 mm when the frame calls for tight positions and the workholding must repeat part to part.
- High precision
- 0.002–0.01 mm on fixtures where the seating error has to be far smaller than the tolerance being verified.
Guidance only: the drawing's value always wins.
Common mistakes when interpreting it
- Reading |A|B|C| as three equal references: it is a sequence, and the primary governs the other two.
- Reordering the datums in the CMM program for probing convenience: the result no longer corresponds to what the drawing requires.
- Assuming the primary datum is always the letter A: the primary is the first one cited in that frame, not the first in the alphabet.
How to get it in the workshop / if it comes out of tolerance
Changing the order changes the measurement. A is primary (usually a plane), B secondary (direction) and C tertiary (origin). Never swap them without good reason.
If two labs disagree on the same part, review the precedence before measuring again. In machining, choose as the primary datum the first surface you prepare and also use it as the clamping face: that way the reference order and the order of operations coincide and you do not stack up error from changing references. If the primary does not seat stably, the problem is its form (flatness, burrs), not the dimension you are measuring.
Related symbols
Other symbols in Datums and references
Related tools
ISO fits
Calculates indicative ISO 286 tolerances for shaft and hole, H7/g6, H7/h6, H7/k6 and H7/p6 fits with clearance, transition and interference graph.
Inspection Plans
Upload a plan, extract dimensions and GD&T with AI and export your inspection guideline in PDF or CSV.
Shop trigonometry
Workshop trigonometry: right triangle, C chamfer, Z countersink, sine bar, taper and polar→X/Y for CNC operators.