GD&T Symbol Guide
Feature Control Frame
The boxed GD&T sentence that names the control, tolerance, modifiers, and datum references for a feature.
Definition
A feature control frame communicates how a geometric tolerance applies to a part feature.
It identifies the GD&T symbol, tolerance value and zone shape, optional material or projection modifiers, and required datum references.
Application
Use the frame anywhere a geometric control is needed on a drawing or model-based definition.
Its datum order matters because primary, secondary, and tertiary references establish the inspection setup in sequence.
Reading A Feature Control Frame
The cells are read left to right: control symbol, tolerance zone, optional modifiers, and datum references.
3D Tolerance Zone
The frame defines the zone shape, size, modifiers, and datum reference frame used for inspection.
Inspection Method
Read the frame from left to right, set up the referenced datums in order, then evaluate the controlled feature inside the specified tolerance zone.
Modifiers such as MMC or LMC change how much tolerance may be available as feature size changes.
Worked Read: Position Callout
A frame shows position, diameter 0.20 at MMC, then datums A, B, and C.
Control
Position
The feature axis is being located.
Zone
Diameter 0.20 at MMC
The base zone is cylindrical and may gain bonus tolerance.
Datums
A | B | C
Inspection constrains A first, then B, then C.
The frame describes a cylindrical position zone built from datum reference frame A-B-C.
Comparison Table
| Frame Cell | Meaning | Example |
|---|---|---|
| Symbol | Geometric control | Position |
| Tolerance | Zone size and shape | Diameter 0.20 |
| Datums | Reference frame | A, B, C |
Notes
Not every control needs a datum; form controls usually stand alone.
The diameter symbol changes many location and orientation zones from planar width to cylindrical diameter.