AI for Design Quality & DFM
The 14 GD&T geometric tolerances, ranked by how often engineers actually call them out, from position and profile down to concentricity and symmetry.
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7 min read
Michelle Ben-David
Michelle Ben-David is a mechanical engineer and Technion graduate. She served in an IDF elite technology and intelligence unit, where she developed multidisciplinary systems integrating mechanics, electronics, and advanced algorithms. Her engineering background spans robotics, medical devices, and automotive systems.
BOTTOM LINE
Learn position, profile of a surface, and flatness first: together they cover most of what shows up on a typical drawing. Perpendicularity, parallelism, and angularity come next, followed by the form and runout controls if your parts rotate. Straightness and profile of a line are specialty tools worth knowing but not worth over-using. Concentricity and symmetry are the outliers on this list: both are still valid under ASME Y14.5, but the 2018 revision pushed designers toward position and profile instead, so today you are more likely to read one of these two on an inherited print than to write one yourself. Compare that inherited callout against the current fits and tolerances guidance before assuming it should stay as written.
Most GD&T references introduce the fourteen geometric characteristics in textbook order: form, orientation, location, profile, then runout. That order is built for teaching categories, not for telling a new engineer where to spend attention. In practice, a working engineer reaches for a handful of these controls every single week and can go a full career barely touching two or three of the others.
This piece ranks all fourteen by how often they actually show up on real drawings, from the three you will tolerance on almost every part to the two you will mostly just read on prints someone else drew years ago. If you are still building GD&T fluency, treat this as a beginner-to-competent path: learn the top of the list first, hold off on memorizing the bottom until you actually need it, and expect the order below to match your own drawing stack more closely than a symbol chart organized by category ever will.
The Three You Will Tolerance on Almost Every Drawing
These three account for most of the geometric tolerancing on a typical mechanical drawing. If you learn nothing else from this list, learn these first.
Position. True position locates holes, pins, bosses, and any other feature that has to line up with a mating part. Nearly every drawing with a bolt pattern or a dowel pin carries at least one position callout, which is why it is the single most common GD&T symbol in practice. It is also the control most engineers learn to exploit for bonus tolerance: reference a hole at maximum material condition and the allowable position tolerance grows as the hole departs from that condition, which is often the difference between a part that requires selective assembly and one that does not.
Profile of a surface. Many shops now use profile as the default control for a functional edge or surface because one symbol, referenced to the right datums, can control form, orientation, and location together. That single-symbol convenience is exactly why it shows up constantly on molded, cast, and sheet metal parts, replacing what used to take three separate callouts stacked on top of each other, and it is the focus of the full symbol chart if you want to see how it compares to the others.
Flatness. Flatness controls the datum features themselves. Because so many parts rely on one broad, flat face as the primary datum for everything else on the print, flatness ends up on the drawing before any of the location or orientation controls that reference it, and an inspector will typically check it first for the same reason. The mechanics of writing that callout, and what it costs to inspect on a surface plate or with a CMM, are covered in detail in this breakdown of flatness.
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The Orientation Controls You Reach for Next
Orientation controls measure how a feature sits relative to a datum rather than controlling the feature against itself, so they only make sense once a datum reference frame is already established.
Perpendicularity. Most mounting faces, bores, and pin holes need to sit square to a datum plane, which makes perpendicularity the most common of the three orientation controls by a wide margin. It is usually verified with a height gauge or CMM sweep against the datum, and it is one of the first checks an inspector runs after confirming flatness on the datum itself.
Parallelism. Close behind, parallelism shows up wherever two faces need to stay an even distance apart, which is most of the time on thickness-critical parts like spacers, plates, and stacked housings, where an out-of-parallel condition on one part telegraphs into every part stacked on top of it.
Angularity. The least common of the three, angularity is reserved for features with a deliberate non-perpendicular, non-parallel relationship to a datum, such as a chamfer, a wedge, or an angled mounting flange on a bracket that has to clear an obstruction at a fixed angle.
The Form and Rotation Controls You Will Use Often, Not Constantly
This group covers turned and rotating features. If your parts are mostly machined plate and sheet metal you may rarely touch these; if you design shafts, bores, or anything that spins, you will use them regularly.
Circularity. Also called roundness, circularity is a routine callout on any turned shaft, bore, or o-ring groove, since it directly governs how well a round cross-section seals or rotates. Shops with a dedicated roundness tester check it as a matter of course on precision-turned parts; without one, a bench comparison against a diameter reading in several orientations is the usual fallback.
Circular runout. Wherever a single cross-section of a shaft or bearing journal has to spin true relative to a datum axis, circular runout is the standard control, and it is common enough to appear on most rotating assemblies. It is checked with a dial indicator held against the feature while the part rotates a full turn on its datum axis.
Total runout. Less common than circular runout, total runout is reserved for surfaces where the entire length, not just one slice, has to run true, such as a sealing surface or a gear hub bore. Verifying it means sweeping the indicator along the full length of the feature while the part rotates, which catches taper and bow that a single circular runout check would miss.
Cylindricity. The least common of the four, cylindricity combines circularity and straightness into a single composite zone. It only earns its place on the drawing when a bearing bore or precision shaft needs tighter form control than circularity or straightness alone can guarantee, and confirming it usually requires a CMM sampling many points around and along the feature rather than a handful of spot checks.
The Controls Reserved for Special Cases
These two still have a job, but a narrower one than the controls above them, so they show up in specific part families rather than across the board.
Straightness. Surface straightness is rare today, mostly replaced by flatness or profile. Axis straightness at maximum material condition is the version that survives, still specified on long pins, shafts, and press-fit features where the bonus tolerance available under the envelope principle actually matters to the fit, such as a dowel pin that needs to slide into a slightly undersized hole without binding.
Profile of a line. A narrower version of profile of a surface, profile of a line controls a single two-dimensional cross-section rather than a full three-dimensional surface. That makes it the right choice for extrusions, sheet metal blanks, and gasket profiles, where checking one representative cross-section is enough to qualify the whole part, but it is called out far less often than its surface counterpart. For a side-by-side look at how the two compare to the rest of the fourteen, see the practical introduction to GD&T.
The Two You Will Mostly Just Read, Not Write, Anymore
Concentricity and symmetry round out the list, and they are the two an engineer today is more likely to encounter on an old print than to specify on a new one.
Concentricity. Concentricity controls the derived median points of a feature relative to a datum axis, and that median-point requirement makes it genuinely difficult to inspect: verifying it properly can call for many individual cross-section measurements rather than a single gage check. ASME Y14.5-2018 flagged this impracticality directly, and most shops now reach for position with two datums, or profile, on new designs instead.
Symmetry. The same median-point problem applies to symmetry, which controls a feature relative to a center plane rather than a center axis. It has largely fallen out of use for the same reason and for the same replacement: position or profile.
Where these two still matter is on inherited drawings, whether from before the 2018 revision or from a supplier still working to the 2009 standard. Opening someone else's print and seeing a concentricity callout raises a real question: was that a deliberate choice, or did it just get copied forward from an older template without anyone revisiting it? That is a knowledge problem as much as a tolerancing one, and it is where the GD&T knowledge gap tends to show up between generations of engineers. Leo AI, an AI intelligence layer that connects to a team's existing PDM and PLM systems, can pull up the current standard alongside the drawing's own revision history and any other prints that used the same feature, so an engineer can check whether a legacy callout was intentional before deciding whether to carry it forward or modernize it.
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