True Position in Geometric Dimensioning and Tolerancing (GD&T) defines a 2D circular or 3D cylindrical tolerance zone within which the center axis or center point of a feature is permitted to vary.
| Symbol | ⌖ True Position |
|---|---|
| Datum Reference | Requires primary, secondary, and tertiary datums (e.g. |A|B|C|) |
| Tolerance Zone Shape | Cylindrical (Diameter ⌀) |
| Verification Instrument | Coordinate Measuring Machine (CMM) or Optical Gauge |
$$\text{Position Deviation } = 2 \times \sqrt{(\Delta X)^2 + (\Delta Y)^2}$$
When the modifier Ⓜ (MMC) is applied, any deviation of the hole size from its smallest allowable diameter generates bonus tolerance, expanding the allowable position error without sacrificing functional assembly.
If a hole specified as 10.0 (+0.2/-0.0) is machined at 10.2mm, it gains 0.2mm of bonus tolerance added directly to the allowable true position zone.
