GD&T symbols chart
All fourteen geometric characteristics — with the two columns most published charts leave out: which controls may take a datum, and which may take a material condition modifier.
| Symbol | Characteristic | Type | Datum | MMC / LMC | What it controls |
|---|---|---|---|---|---|
| ⏤ | Straightness | Form | Never | Yes* | How much a line element or an axis may bow away from perfectly straight. |
| ⏥ | Flatness | Form | Never | Yes* | All points of a surface must lie between two parallel planes the tolerance apart. No datum — the surface is judged against itself. |
| ○ | Circularity | Form | Never | No | Each individual cross-section must lie between two concentric circles the tolerance apart. A slice-by-slice control, not a whole-feature one. |
| ⌭ | Cylindricity | Form | Never | No | The whole surface must lie between two coaxial cylinders the tolerance apart. Controls roundness, straightness and taper simultaneously. |
| ∠ | Angularity | Orientation | Required | Yes* | The feature must lie within a zone at the specified basic angle to the datum. Controls orientation only, not location. |
| ⊥ | Perpendicularity | Orientation | Required | Yes* | The feature must sit within a zone exactly 90° to the datum. It says nothing about where the feature is, only how it is oriented. |
| ∥ | Parallelism | Orientation | Required | Yes* | The feature must lie within a zone parallel to the datum. Again orientation only — it does not fix the distance. |
| ⌖ | Position | Location | Required | Yes* | The feature axis or centre plane must lie within a zone centred on the theoretically exact (basic) location. The workhorse of GD&T. |
| ◎ | Concentricity | Location | Required | No | The derived median points of the feature must lie within a cylindrical zone about the datum axis. Note this is about median POINTS, not the surface. |
| ⌯ | Symmetry | Location | Required | No | The derived median points of a feature must lie within a zone about the datum centre plane. The planar analogue of concentricity. |
| ⌒ | Profile of a Line | Profile | Optional | No | Each 2D cross-section of the outline must lie in a zone about the true profile. Without datums it controls form only; with them, also orientation and location. |
| ⌓ | Profile of a Surface | Profile | Optional | No | The whole surface must lie in a 3D zone about the true profile. With datums it controls form, orientation and location at once, which is why it can replace several other callouts. |
| ↗ | Circular Runout | Runout | Required | No | Full indicator movement at a single cross-section as the part is rotated about the datum axis. Combines circularity and coaxiality in one reading. |
| ⌰ | Total Runout | Runout | Required | No | Indicator movement across the entire surface while rotating about the datum axis. Adds cylindricity and taper control to circular runout. |
* The material condition rules, precisely. Position takes MMC or LMC freely. Straightness takes it only on a derived median line (an axis), and flatness only on a derived median plane — both are features-of-size cases. Perpendicularity, angularity and parallelism take it only when the toleranced feature is a feature of size, never on a plain surface. Profile never takes it on the tolerance value, although a datum letter in the same frame still can — that is datum shift, not bonus tolerance. Runout never takes it anywhere, not even on the datum reference.
The four form controls never take a datum — straightness, flatness, circularity and cylindricity all judge a feature against itself.
ASME vs ISO, the difference that matters. ASME Y14.5 applies Rule #1 silently: perfect form is required at MMC on every feature of size unless you say otherwise (circled I switches it off). ISO 8015 defaults to the opposite — independency, where size and form are separate, and you must add circled E to require the envelope. The same drawing means different things under each.
Edition note. ASME Y14.5-2018 withdrew concentricity and symmetry, taking the official count from 14 to 12. Both remain in Y14.5-2009 and ISO 1101 and still appear on drawings, so they are listed here — but new designs should use position or runout instead.
Decode a full feature control frame with the GD&T symbol decoder.
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