GD&T Quick Reference
The fourteen geometric characteristic symbols, grouped by what they actually control on a print.
Form
| Characteristic | What it controls |
|---|---|
| Straightness | How much a line element can wander from a perfectly straight line. |
| Flatness | How much a surface can vary from a perfectly flat plane. |
| Circularity (roundness) | How close a cross-section is to a true circle, at any single slice. |
| Cylindricity | Circularity, straightness, and taper all controlled together over a full cylinder. |
Orientation
| Characteristic | What it controls |
|---|---|
| Angularity | A surface or axis held at a specified angle other than 0° or 90° to a datum. |
| Perpendicularity | A surface or axis held at exactly 90° to a datum. |
| Parallelism | A surface or axis held equidistant from a datum along its length. |
Location
| Characteristic | What it controls |
|---|---|
| Position | Where a feature's center, axis, or plane falls relative to datums — the most commonly used GD&T callout. |
| Concentricity | Whether a feature's median points share the same center as a datum axis. |
| Symmetry | Whether a feature's median points are evenly split across a datum plane. |
Profile
| Characteristic | What it controls |
|---|---|
| Profile of a line | A tolerance zone around a single 2D cross-section of a surface. |
| Profile of a surface | A tolerance zone around an entire 3D surface, often used with or without datums. |
Runout
| Characteristic | What it controls |
|---|---|
| Circular runout | Wobble at a single cross-section as a part rotates around a datum axis. |
| Total runout | Circular runout plus surface variation checked across the entire feature at once. |
Position is worth learning first
Position tolerancing, usually applied to holes and other features of size, is by far the most common GD&T callout on a typical print and the one most worth understanding cold — it replaces separate X/Y coordinate tolerances with a single circular (or cylindrical) tolerance zone, which is both more forgiving to manufacture to and more representative of how a mating part actually needs to line up.
Datums give every symbol its reference frame
Almost every symbol on this chart means nothing without a datum reference to measure against — the datum letters (A, B, C…) called out in a feature control frame define the actual coordinate system the tolerance zone is built from. Reading the datum reference frame correctly matters as much as reading the symbol itself.
This is a memory aid, not a full GD&T course
ASME Y14.5 (the full GD&T standard) covers material condition modifiers, bonus tolerance, datum precedence, and plenty of nuance this chart doesn't attempt to replace — use this to jog your memory on what a symbol means, not as a substitute for the standard itself on a critical print.