Read a feature control frame from left to right: symbol, tolerance value, then any datum references. Form controls are self-contained and do not use datums. Orientation, location and runout controls are datum-relative. Profile controls may be datum-relative when the profile must be located or oriented to the rest of the part.
Many shop searches call these geo tolerance symbols or geometric tolerance symbols. In drawing-standard language, they are GD&T geometric characteristic symbols.
The symbol alone is not the full requirement. A diameter sign creates a cylindrical zone, a material-condition modifier changes how bonus tolerance is earned, and the datum order fixes how the part is seated for inspection. For worked examples, use the GD&T symbols guide; for a numerical position check, use the true position calculator.
| Characteristic | Symbol | Family | Datum? | Typical tolerance zone |
|---|---|---|---|---|
| Straightness | ⏤ | Form | No | Two parallel lines, or a cylinder for an axis when the diameter symbol is used |
| Flatness | ⏥ | Form | No | Two parallel planes that contain the entire controlled surface |
| Circularity | ○ | Form | No | Two concentric circles in each checked cross-section |
| Cylindricity | ⌭ | Form | No | Two coaxial cylinders around the whole controlled surface |
| Profile of a line | ⌒ | Profile | Usually | Two offset curves around the true profile in a single section |
| Profile of a surface | ⌓ | Profile | Usually | Two offset surfaces around the true 3D profile |
| Angularity | ∠ | Orientation | Yes | Two parallel planes or a cylinder at the specified basic angle |
| Perpendicularity | ⊥ | Orientation | Yes | Two parallel planes or a cylinder exactly 90 degrees from the datum |
| Parallelism | ∥ | Orientation | Yes | Two parallel planes or a cylinder parallel to the datum |
| Position | ⌖ | Location | Yes | A cylinder or paired planes located from basic dimensions and datums |
| Concentricity | ◎ | Location | Yes | A cylindrical zone whose median points share the datum axis |
| Symmetry | ⌯ | Location | Yes | Two parallel planes centered on the datum center plane |
| Circular runout | ↗ | Runout | Yes | A circular band checked one section at a time during rotation |
| Total runout | ⌰ | Runout | Yes | A full surface band checked over the whole controlled surface during rotation |
Source: ASME Y14.5 geometric characteristic families and common drawing-practice interpretations. Verify the controlling drawing standard and company notes for production release.
Material condition and datum modifiers
The symbols above identify the geometric characteristic. A feature control frame can also include modifier symbols such as MMC (maximum material condition), LMC (least material condition) or RFS (regardless of feature size, normally implied by no symbol). In a position callout, MMC can add bonus tolerance as a hole gets larger or a pin gets smaller; the MMC bonus tolerance calculator handles that arithmetic directly, and the maximum material condition guide explains the material condition modifier and virtual condition boundary.