Guide
Symmetry GD&T was removed in ASME Y14.5-2018: should you replace it with position or profile?
Symmetry GD&T is the old ASME location control that required the median points of opposed surface elements to lie in a zone centered on a datum center plane; ASME Y14.5-2018 removed the term, the symbol and the concept. Replace it with position when the feature is a slot, tab or keyway with parallel sides that you want to gauge, and with profile of a surface when the sides are not a simple width or each surface needs its own control.
By Saola Manufacturing engineering team · Published · Updated
The full list of 2018 changes, including the removal of concentricity, is in the ASME Y14.5-2018 changes guide. This page deals only with symmetry: what the old symbol actually required, how to rewrite it, and how the rewrite changes inspection.
On this page
What did the symmetry symbol actually control?
Under ASME Y14.5-2009, symmetry controlled the median points of all opposed elements of the toleranced surfaces, which had to fall within two parallel planes centered on the datum center plane. It applied regardless of feature size, so it gave no bonus tolerance and could not be checked with a fixed gauge.
The Mitutoyo Institute of Metrology summary of the 2009 definition reads: symmetry is the condition where the median points of all opposed elements of two or more feature surfaces are congruent with a datum axis or center plane (Mitutoyo, GD&T and the new ASME Y14.5-2018, 2019). In other words, it was not about the center plane of the slot as a whole. It was about every pair of opposite points, one on each wall, and the midpoint between them.
That definition made symmetry expensive to verify. An inspector has to sample many opposed point pairs, compute each midpoint and compare it with the datum plane. GD&T Basics describes the usual method as a CMM set up to compute the theoretical midpoint plane, and calls it complex and sometimes inaccurate; the same page notes that MMC and LMC do not apply to symmetry (GD&T Basics, Symmetry, 2014).
Why did ASME remove symmetry in the 2018 edition?
ASME removed concentricity and symmetry to eliminate the confusion around these symbols and their misapplication; many organizations had banned them long before. The 2018 edition, reaffirmed in 2024 and now on stabilized maintenance, has no symmetry symbol at all.
The Mitutoyo summary quotes the reason given, to eliminate the confusion that surrounds these symbols and their misapplication, and lists the change as removal of the term, the symbol and the concept (Mitutoyo, 2019). ASME lists Y14.5-2018 as replacing the 2009 edition, carrying a 2024 reaffirmation and placed on stabilized maintenance (ASME Y14.5 product page).
Two consequences follow for a buyer. A drawing that cites Y14.5-2018 and still shows the symmetry symbol uses a control the cited edition does not define, so a careful supplier will query it. And a drawing made to ISO GPS standards may still use symmetry legitimately, because ISO 1101 keeps a symmetry tolerance with its own definition; the ASME vs ISO GPS guide covers that difference.
Position or profile: which replacement fits which feature?
Use position when the controlled feature is a feature of size with two parallel opposed surfaces, such as a slot, tab or keyway, and you want MMC and gauging. Use profile of a surface when the surfaces are not a simple width, when each side needs its own limit, or when size and location should be one zone.
The table sorts the common cases. In every row the datum is still the center plane you used for symmetry, typically established by the two outside faces of the part or a datum width.
| Feature situation | Replacement | What it controls | How it is usually inspected |
|---|---|---|---|
| Slot, tab or keyway that must assemble with a mating part | Position at MMC to the datum center plane | Center plane of the feature, with bonus tolerance as the width departs from MMC | Functional gauge or a computed position value |
| Slot or tab whose centering matters at any size (balance, symmetry of mass) | Position at RFS | Center plane of the actual mating envelope, no bonus | Vision system, height gauge on a surface plate, or CMM |
| Two surfaces that are not parallel, such as a V-groove or tapered tab | Profile of a surface with basic dimensions from the center plane | Each surface within its own zone; controls location and form together | Point checks against basic dimensions |
| Width and centering that you want held as one zone | Profile of a surface applied to both sides (all around the pair) | Size, location and form in one requirement | Point checks or scan |
| Feature already controlled by its mating gauge | Position at MMC with a stated gauge | Same as the gauge, in drawing language | The gauge itself |
How do you rewrite the callout on the drawing?
Keep the width dimension with its tolerance, change the symbol to position (with MMC if assembly is the function) and keep the same datum center plane. For profile, replace the width tolerance with basic dimensions from the center plane and a profile tolerance on the surfaces.
A typical rewrite for a keyway or slot: leave the slot width as a size tolerance, then add a feature control frame with the position symbol, the tolerance value, the MMC modifier if wanted, and the datum letter of the center plane. The position zone is two parallel planes centered on the datum plane, and at MMC the zone grows as the slot opens up, which is usually what the mating key or tab needs.
Do not copy the old symmetry tolerance value blindly. Symmetry was evaluated on median points, which scatter more than a single envelope center plane, so the same number under position is often a little looser in effect. If the function was assembly, work the value out from the mating part, as the MMC bonus tolerance guide shows. For a profile rewrite, define each surface with basic dimensions from the datum center plane; the profile guide explains the zone.
On a revised print, add a revision note saying the symmetry callout was replaced and why, so the supplier knows the old inspection method no longer applies and does not keep reporting median-point data.
How does inspection change after the rewrite?
Position at MMC can be checked with a simple functional gauge, and position at RFS or profile can be checked on a vision system, a surface plate with a height gauge, or a CMM. None of them needs the point-pair median calculation symmetry required.
For a slot or tab on a milled part, the common shop method for position at RFS is to measure both walls from each datum face, compute the slot center plane and compare it with the datum center plane. A 2D vision system does this well on profiles that can be seen in silhouette or from above; a height gauge on a surface plate does it on parts that can be flipped against the datum faces. For position at MMC, a gauge with a fixed-width blade located on the datum faces accepts every part that can assemble.
Agree in the RFQ how the result will be reported: a computed position value per feature is clearer than a table of raw coordinates. The feature control frame guide covers how to read the frame you end up with.
How does Saola handle a symmetry callout on a drawing?
A symmetry symbol on an ASME 2018 drawing is raised at drawing review, with a suggested position or profile rewrite for you to approve. Nothing is reinterpreted silently.
Slots, keyways and tabs are cut on the lead plant's CNC milling and CNC turning lines, and the inspection equipment on the quality page includes a 2D optical measuring instrument and a height gauge. There is no coordinate measuring machine, so a callout that can only be verified on one is also flagged at review, with a gauge or vision-based alternative where one exists.
If your drawing is to ISO GPS and uses symmetry under ISO 1101, say so in the RFQ so it is read under that standard. Send the drawing for a quote and mark which callouts were carried over from older revisions.
Frequently asked
Is symmetry the same as position at RFS?
Not exactly. Symmetry controlled the midpoints of opposed point pairs; position controls the center plane of the feature's actual mating envelope. On a well-formed slot the results are close, but on a slot with poor wall form they can differ.
Can I still use the symmetry symbol if my drawing cites the 2009 edition?
Yes, the 2009 edition still defines it, and a drawing is read under the edition it cites. But it is costly to verify and often misapplied, so replacing it removes the ambiguity.
Does position need the MMC modifier?
No. Use MMC when the function is assembly and you want bonus tolerance and gauging; use RFS when centering matters at every size.
What replaced concentricity?
Position, runout or profile, depending on function. The concentricity vs runout vs position guide covers that choice.
Do I have to update old drawings?
Only when you revise them or move them to a new supplier. That is the right moment to replace symmetry, because the new supplier will query it anyway.
Sources
- GD&T and the new ASME Y14.5-2018, Jim Salsbury, Mitutoyo Institute of Metrology, Mitutoyo America Corporation, 2019
- ASME Y14.5-2018 (R2024), Dimensioning and Tolerancing (product page), ASME, accessed November 2026
- Symmetry, GD&T Basics, Tom Geiss, November 2014