Engineering quick reference
GD&T Symbol Reference
The 14 geometric tolerance characteristics — symbol, category, datum requirement, and plain-English definition (ASME Y14.5).
How to read a feature control frame — true position Ø0.010 at MMC, datums A | B | C
Key: Req = datum reference required · Opt = datum optional · — = none · Ⓜ MMC · Ⓛ LMC · no modifier = RFS · FIM = full indicator movement
| Sym | Characteristic | Datum | Definition |
|---|---|---|---|
| FormNo datum — feature judged against itself | |||
| — | Straightness | — | Surface line elements must lie between two parallel lines; when applied to an axis (derived median line, Ø in the frame), the axis must lie within a cylindrical zone. |
| ⏥ | Flatness | — | All points on a planar surface lie within a tolerance zone bounded by two parallel planes. |
| ○ | Circularity (roundness) | — | At any cross-section, the surface must lie between two concentric circles separated by the tolerance value. |
| ⌭ | Cylindricity | — | Combines straightness, circularity, and taper into one tolerance — the whole cylindrical surface lies between two coaxial cylinders. |
| OrientationDatum required | |||
| ∥ | Parallelism | Req | A surface or axis must be parallel to a referenced datum within the tolerance zone. |
| ⟂ | Perpendicularity | Req | A surface or axis must be perpendicular to a referenced datum within the tolerance zone. |
| ∠ | Angularity | Req | Surface or axis at a specific basic angle to a datum, within the tolerance zone. |
| LocationDatum required | |||
| ⌖ | Position (true position) | Req | An axis or feature center must lie within a cylindrical or other-shape tolerance zone at the basic position relative to datums. |
| ◎ | Concentricity | Req | Median points of a feature lie within a cylindrical zone coaxial with a datum axis. Withdrawn in ASME Y14.5-2018 (still seen on legacy Y14.5-2009 drawings) — replace with position-of-axis or runout. |
| ⌯ | Symmetry | Req | Median points of feature lie within a planar zone symmetric to a datum plane. Withdrawn in ASME Y14.5-2018 (still seen on legacy Y14.5-2009 drawings) — replace with position. |
| RunoutDatum axis required — checked while rotating | |||
| ↗ | Circular runout | Req | FIM (full indicator movement) at any single cross-section as the part rotates about a datum axis. |
| ⌰ | Total runout | Req | FIM across the entire surface as the part rotates and the indicator traverses the surface — axially along a diameter, radially across a face perpendicular to the axis. |
| ProfileDatum optional | |||
| ⌒ | Profile of a line | Opt | A line element of a surface lies within a 2D tolerance zone defined by basic dimensions. Used for 2D profiles like a cross-section. |
| ⌓ | Profile of a surface | Opt | A 3D surface lies within a 3D tolerance zone defined by basic dimensions. Most commonly applied to complex curved or contoured surfaces. |
ASME Y14.5-2018 defines 12 current geometric characteristics. Concentricity andsymmetry were withdrawn in the 2018 revision (still found on legacy Y14.5-2009 and earlier drawings) — replace concentricity with position-of-axis or runout, and symmetry with position. Feature control frame reads left to right: symbol · tolerance (with Ø for a cylindrical zone) · material modifier (Ⓜ MMC / Ⓛ LMC / none = RFS) · datum references A B C. For worked examples and datum guidance, see the fullGD&T symbols reference.
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