GD&T Symbol Guide
True Position
Controls how far a feature of size may drift from its theoretically exact location, usually from a datum reference frame.
Definition
Position defines the allowed variation between a feature's actual derived center, axis, or median plane and its theoretically exact location.
The exact location is established with basic dimensions, then locked to a datum reference frame so the drawing describes function rather than a chain of plus/minus coordinates.
Application
Use it for holes, pins, slots, tabs, bosses, and other features of size where assembly depends on location.
With MMC, the callout can allow bonus tolerance as a clearance hole grows larger than its maximum material condition.
Hole Pattern Located From Datums A, B, and C
The basic dimensions locate the true position. The feature control frame gives the permitted cylindrical zone for each hole axis.
3D Tolerance Zone
Most commonly a cylindrical zone centered on the true location of the feature axis.
Inspection Method
Measure the feature center in X and Y from the datum setup, then convert the radial offset into a diametric positional error.
For a hole, also check along the depth so tilt of the axis is caught, not just the center at one surface.
Worked Check: Off-Center Bolt Hole
A clearance hole is specified at true position with diameter 0.20 at MMC. Inspection finds the hole center is 0.06 mm off in X and 0.04 mm off in Y.
Radial offset
sqrt(0.06^2 + 0.04^2) = 0.072 mm
The center is 0.072 mm away from the exact basic location.
Diametric error
2 x 0.072 = 0.144 mm
Position is usually reported as a diametric error.
Compare
0.144 <= 0.20
The axis passes before any MMC bonus tolerance is needed.
The hole location passes. If the actual hole is larger than MMC, the available position tolerance would increase.
Comparison Table
| Control | Best For | Datum Required | Zone Shape |
|---|---|---|---|
| Position | Locating holes, pins, slots, bosses | Usually yes | Cylindrical or planar |
| Perpendicularity | Keeping an axis square to a datum | Yes | Cylindrical or planar |
| Profile | Locating and shaping surfaces | Often | Boundary around a surface |
Notes
A diameter symbol gives a round or cylindrical tolerance zone. Without it, the zone behaves like a width between parallel lines or planes.
Position can also control orientation because the feature axis must stay inside the zone through the part thickness.