Day one of five. The GD&T guide covering this change is free on Kindle, with three other engineering books, until Tue 10 Nov 23:59 PT. Time remaining: ends Tue 10 Nov, 11:59 pm PT.
This explainer was published during a free Kindle week for four engineering books in November 2026.
For decades, a designer who wanted a shaft centred in a bore could reach for concentricity. One who wanted a slot centred on a block could use symmetry. Both symbols are gone from ASME Y14.5-2018, and the reason says a lot about how tolerancing has matured.
What the symbols really asked for
Both controls sound simple, but neither meant what most people assumed. They did not directly control a feature's surface or its axis. They controlled the median points of a feature, the midpoints between opposite surface points, and every one of those midpoints had to lie within the zone.
Measuring that properly means capturing many points, pairing them and calculating each midpoint. Few shops did so. Many inspected these features using simpler methods such as runout or position, which meant the number on the report didn't quite match the requirement on the drawing.
What replaces them
The standard already had better tools for most jobs.
- Position controls the location of a feature of size, and it can use material condition modifiers, which symmetry and concentricity could not.
- Runout suits rotating parts, because it checks the surface as the part spins about a datum axis.
- Profile can control a surface's location and form together.
So the usual question becomes what the function actually requires. A rotating seal surface points to runout. A keyway that must assemble with a key points to position.
Older drawings still exist
Removing a symbol doesn't erase drawings. A drawing that cites an earlier edition stays under that edition, so engineers, suppliers and inspectors will meet these symbols for years. Anyone updating a legacy drawing to the 2018 standard should treat it as a design decision, not a find-and-replace job, since the replacement control is not identical in meaning.
For the basics behind these controls, see our plain-English guide to GD&T.
The four books
GD&T Professional Exam Guide
Practical Airframe Stress Analysis, Volume 1: Metallic Structure
Practical Airframe Stress Analysis, Volume 2: Composite Structure
Python for Aerospace Stress Engineers (Volume 3)
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