GD&T Symbol Guide
Envelope Requirement
An ISO symbol that makes size control form through the perfect-form envelope at MMC.
Definition
Envelope requirement is an ISO drawing symbol that states size controls form at the perfect-form boundary of MMC.
It produces an effect similar to ASME Rule #1, which is not the ISO default.
Application
Use it on ISO drawings when a feature of size must not violate the maximum-material perfect-form envelope.
It can be applied to an individual dimension or through a general note when the whole drawing needs the principle.
ISO Envelope Requirement Applied To Size
ISO drawings use the circled E when size is intended to control form like ASME Rule #1.
3D Tolerance Zone
The feature must remain within the perfect-form envelope at maximum material condition.
Inspection Method
Check local sizes and verify the feature surface remains inside the MMC perfect-form envelope.
Use separate form controls when a tighter or more specific form limit is needed.
Worked Read: ISO Pin
An ISO pin dimension includes the circled E symbol with size limits 9.90 to 10.00 mm.
MMC
10.00 mm
Largest pin contains the most material.
Envelope
Perfect cylinder at 10.00
The surface must not violate this boundary.
Local size
Within limits
Every local size must still remain between 9.90 and 10.00 mm.
The ISO callout applies envelope behavior to the pin.
Comparison Table
| Symbol | Meaning | Where Common |
|---|---|---|
| E | Envelope requirement | ISO drawings |
| Rule #1 | Envelope by default | ASME drawings |
| I | Independency | ASME exception |
Notes
ASME generally assumes the envelope principle by default for regular features of size.
ISO generally treats size and form independently unless the envelope requirement is shown.