GD&T (geometric dimensioning and tolerancing) tells the shop floor what really matters on a part. Without it, a drawing is ambiguous — two inspectors can both pass and reject the same piece.
The seven symbols you’ll see most
- Flatness — surface within two parallel planes
- Parallelism — surface parallel to a datum within a tolerance zone
- Perpendicularity — 90° to a datum within a tolerance zone
- Position — feature center within a circular zone around true position
- Concentricity — axis aligned with a datum axis
- Runout — combined effect of form, location, orientation in rotation
- Profile — surface conforms to a defined shape within bilateral tolerance
Reading the feature control frame
⏥ 0.002 A │ │ │ │ │ └─ datum (reference surface A) │ └────── tolerance value (0.002") └────────── geometric symbol (flatness)
Read left to right. Symbol, then tolerance, then datum references. That’s the entire grammar.