GD&T Symbol Guide
GD&T Rule #1
The default ASME rule that limits feature form by the perfect-form envelope at maximum material condition.
Definition
Rule #1 says the form of a regular feature of size is controlled by its limits of size.
At maximum material condition, the actual surface cannot extend beyond the perfect-form envelope.
Application
Use it as the default interpretation for ASME drawings unless an exception or separate form control changes the requirement.
It helps ensure holes, pins, tabs, and slots remain assembleable even when only size limits are shown.
Size Limits Controlling Form
At MMC the envelope requires perfect form; as size departs from MMC, more form variation can exist within the envelope.
3D Tolerance Zone
The feature surface may not violate the perfect-form boundary at MMC while local sizes remain within limits.
Inspection Method
Check local sizes against the dimensional limits, then verify the feature does not violate the MMC perfect-form boundary.
Add flatness, straightness, circularity, or cylindricity when the default envelope does not control form tightly enough.
Worked Check: Pin Envelope
A pin has size limits 9.90 to 10.00 mm. At 10.00 mm MMC, its surface must fit a perfect 10.00 mm cylinder.
MMC
10.00 mm
Largest pin size contains the most material.
Envelope
Perfect cylinder at 10.00
No surface point may break this boundary.
Departure
Smaller local size
Form may vary only while the envelope is still respected.
The pin must satisfy both local size limits and the perfect-form boundary.
Comparison Table
| Concept | ASME Default | ISO Default |
|---|---|---|
| Size controls form | Yes via Rule #1 | Only with envelope requirement |
| Independency | Exception | Default principle |
| Refining form | Add form controls | Add form controls |
Notes
Rule #1 is also called the envelope principle or Taylor principle.
Exceptions include independency, free-state requirements, stock dimensions, average diameter, and explicit form controls on features of size.