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All 14 GD&T symbols from ASME Y14.5, sorted into form, orientation, location, profile, and runout, with the datum rules and tolerance zone for each.
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8 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
Fourteen symbols, five families, one distinction that decides everything: form controls need no datum and check a feature against itself, while orientation, location, profile, and runout all measure a feature against something else. Picking the wrong family costs more than a redline on a drawing review. A perpendicularity callout with no material condition modifier, a concentricity spec nobody can put on a CMM, or a profile zone measured from the wrong datum all turn into scrapped parts and inspection reports nobody trusts. Start with the question the feature is actually asking, shape, angle, location, or spin, and the symbol usually picks itself. When it does not, that is the signal to check how the same feature was handled on a related part rather than guess.
Every geometric dimensioning and tolerancing callout on a drawing traces back to one of fourteen symbols defined in ASME Y14.5. Engineers who only half remember them tend to over-tolerance a part, pick a symbol that needs a datum it never gets, or specify a control nobody in the shop can actually measure. This chart sorts all fourteen into the five families the standard groups them into: form, orientation, location, profile, and runout. Each family answers a different question about a feature, and once you know which question you are asking, picking the right symbol stops being a memorization exercise.
None of this is exotic knowledge. Most of the fourteen symbols show up on a typical machined part drawing at some point, and a reviewer who cannot tell a form control from an orientation control at a glance will either wave through a callout that cannot be inspected, or reject one that was correct all along. The families below are ordered the way a designer should actually think through a feature: is this shape check standalone, or does it depend on something else on the part.
Two of the fourteen already have their own deep dives on this site. Flatness gets a full breakdown of callout wording and inspection cost, and the GD&T knowledge gap post covers why these mistakes keep recurring on drawings from otherwise experienced teams. Use this page as the map, and the linked posts for the detail.
Form Tolerances: How a Feature Behaves on Its Own
Straightness, flatness, circularity, and cylindricity make up the form family, and they share one trait: none of them ever reference a datum. A form control asks whether a single feature holds its own shape, independent of anything else on the part.
Straightness restricts how much a line element or an axis is allowed to bow. Applied to a surface it controls individual line elements; applied to a feature of size with the diameter modifier it controls the derived median line, which is really an indirect way of holding a bent-shaft problem in check.
Flatness holds every point on a surface between two parallel planes separated by the tolerance value. It has no length or width limit of its own, which is the detail covered in the flatness breakdown linked above.
Circularity (roundness) holds each circular cross-section, taken independently, between two concentric circles. It says nothing about how those cross-sections line up with each other along the axis.
Cylindricity is circularity's three-dimensional sibling. It controls an entire cylindrical surface, roundness, straightness, and taper, all at once, inside one annular tolerance zone.
Because form controls need no datum, they are also the cheapest to inspect: a surface plate, a height gauge, and a bit of patience will verify flatness. The moment a drawing calls out a form control with a datum reference, that is worth flagging. It usually means the designer meant to call out an orientation control instead.
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Orientation Tolerances: Where One Feature Sits Relative to Another
Angularity, perpendicularity, and parallelism all answer the same question at a different angle: how does this feature sit relative to a datum. Every orientation control needs at least one datum reference in its feature control frame, and the tolerance zone is still two parallel planes, or, for an axis, a cylinder, just tilted to whatever angle the datum demands.
Perpendicularity holds a surface or axis at 90 degrees to a datum, within the stated zone. It is the control most often confused with flatness, because a surface that fails flatness will usually fail perpendicularity too, though the reverse is not guaranteed.
Parallelism is perpendicularity's zero-degree case: the feature has to stay within two planes parallel to the datum. It shows up constantly on mounting faces and bearing bores that have to line up with a reference surface on assembly.
Angularity covers everything that is not zero or 90 degrees, a chamfer relative to a bore axis, a wedge face relative to a base. The math does not change; only the angle in the basic dimension does.
A tolerance zone this size only matters when it is measured against the right datum in the right order, which is where the calculations in fits and tolerances start to interact with GD&T: an orientation callout defines the zone, but the fit class decides whether that zone actually leaves room for the mating part.
Location Tolerances: Position, and the Two Symbols Standards Are Phasing Out
Position, concentricity, and symmetry are the location family. All three need datums, and all three define where a feature is allowed to sit relative to a datum reference frame rather than what shape it has to hold.
Position is the workhorse of the group. It defines a cylindrical, or for a slot, a boundary, tolerance zone centered on a true position taken from basic dimensions, and it is almost always paired with a material condition modifier so the zone can grow as the feature departs from its worst-case size.
Concentricity controls the median points of a feature relative to a datum axis. ASME Y14.5-2018 keeps the symbol in the standard but no longer defines a practical inspection method for it, which is why most current drawings replace it with a position or runout callout that can actually be verified on a CMM.
Symmetry has the same problem: it is defined against the median points of two opposed features, and in practice a profile or position control does the same job with an inspection path that exists.
Datum reference frame order, primary, secondary, tertiary, is where position callouts go wrong more than anywhere else on the drawing, and it is rarely a symbol problem. It is a "which face did we actually mean" problem. Leo sits on top of an organization's PDM and drawing history rather than replacing it, so when an engineer is choosing a datum scheme, it can surface how the same feature was toleranced on a prior revision or a related part instead of leaving that decision to memory.
Profile Tolerances: One Symbol, Three Jobs
Profile of a line and profile of a surface are the most flexible controls in the standard, because depending on how many datums are referenced, a profile callout can control form alone, form and orientation, or form, orientation, and location all at once.
Profile of a line defines a tolerance zone as two offset curves surrounding the true profile at any single cross-section. It is used far less often than profile of a surface, mostly on parts where only one cross-section actually matters.
Profile of a surface wraps the entire true profile in a three-dimensional zone, most commonly split evenly on either side of the basic profile, though it can be applied unequally when a designer needs more room on one side.
This is why profile has been replacing position on complex, non-planar surfaces: castings, sheet metal parts, cam profiles, anywhere a single feature control frame covering the whole shape is cheaper to program into a CMM than five or six separate callouts. The tradeoff is that profile is unforgiving of a poorly chosen datum reference frame, which loops right back to the datum sequencing problem in the location section above.
Runout Tolerances: Built for Anything That Spins
Circular runout and total runout only make sense on rotating parts: shafts, hubs, anything measured while spinning on a fixture indexed to a datum axis.
Circular runout checks a single circular element at a time as the part rotates a full 360 degrees around the datum axis, catching out-of-round and axis offset at that one cross-section.
Total runout sweeps an indicator across the entire surface while the part rotates, catching everything circular runout catches plus taper, straightness, and flatness variation along the length. It is the strictest control in the standard and the most expensive to inspect, since it needs a full surface sweep rather than a handful of cross-sections.
Both controls need a datum axis, usually built from two datum diameters or a diameter and a face, and both belong on the same feature control frame convention the rest of the drawing uses. A chart like this one only pays off if the callouts it describes are applied the same way from one drawing to the next, which is the whole argument behind keeping drawing standards consistent across a team in the first place. Stack-up problems that slip through inconsistent runout callouts are exactly what tolerance stack-up analysis catches downstream, after the parts are already in the assembly.
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