GD&T Symbol Guide
Independency
An ASME modifier that separates size control from form control for the associated feature.
Definition
Independency indicates that the requirement for perfect form at MMC is removed for the associated size dimension.
Size and form are then controlled separately, usually with another form control if form still matters.
Application
Use it only when the default Rule #1 envelope is not intended for a feature of size.
It is an ASME concept; ISO uses independency as the default unless envelope requirement is added.
Size And Form Controlled Separately
The independency symbol removes the default simultaneous size-and-form control for that feature.
3D Tolerance Zone
Size limits and form limits are evaluated separately instead of through the Rule #1 envelope.
Inspection Method
Check size against the stated limits, then inspect any separate form control independently.
Without a separate form control, the feature form may be much less constrained than under Rule #1.
Worked Read: Plate Thickness
A plate thickness has an independency symbol and the lower face has a separate flatness callout.
Size
Check thickness limits
Thickness is accepted by local size only.
Envelope
Removed
Perfect form at MMC is not imposed by Rule #1.
Form
Check flatness
The separate flatness callout controls the named surface.
The drawing separates thickness acceptance from the surface flatness requirement.
Comparison Table
| Concept | Effect | Standard Context |
|---|---|---|
| Rule #1 | Size controls form | ASME default |
| Independency | Size separate from form | ASME exception |
| Envelope requirement | Size controls form | ISO addition |
Notes
Independency can increase manufacturing freedom but must be used carefully around assembly features.
It is often paired with a refining flatness, straightness, circularity, or cylindricity requirement.