GD&T concept · Rule

Independency (Ⓘ) — Opting Out of Rule #1

The Ⓘ modifier that removes Rule #1's envelope — size and form become independent requirements.

Definition

Independency (Ⓘ) explicitly removes Rule #1's envelope requirement from a feature of size. Size and form become independent requirements: the size must stay within its stated limits, and the form is whatever the actual surface happens to be — the MMC envelope imposes nothing. Under ASME Y14.5 the Ⓘ symbol is placed in the tolerance compartment of the feature control frame, right where Ⓜ or Ⓛ would go; earlier revisions invoked the same principle by drawing note.

The envelope removed

With the envelope gone, a feature can be at or near its MMC size and still carry form error: a Ø10 ±0.1 pin produced at Ø9.95 may bow by more than the 0.05 that Rule #1 would have allowed — even past the Ø10 boundary — as long as its measured size stays within limits and its form does not violate any form tolerance the drawing adds. The surface is allowed to poke through the MMC boundary because there is no boundary; the only requirements are the ones explicitly written.

With independency the envelope is removed: the actual shaft's form error may exceed the MMC boundary while its size stays within limits — size and form are checked as independent requirements. Schematic, exaggerated.

The default inversion, ASME vs. ISO

This is the mirror image at the heart of the ASME/ISO difference. ASME defaults to Rule #1 — the envelope is in force unless Ⓘ says otherwise. ISO defaults to independence — form and size vary freely unless Ⓔ (the envelope requirement) says otherwise. Same two ideas, opposite defaults: ASME draws the opt-out, ISO draws the opt-in. A drawing interpreted under the wrong standard silently gains or loses its envelope, which changes what parts pass inspection.

When independency is used

Flexible parts — stampings, seals, thin sheet — where an envelope would be impossible or meaningless to hold
Features whose form is controlled separately with a real form tolerance, so the envelope would be redundant and double-counted
Long, slender parts where Rule #1 would force a straightness the function does not need
Parts that run with deliberate waviness — bearing and seal surfaces where the texture is part of the design

The cost of opting out

Rule #1 is usually free: it guarantees the assembly fit that MMC callouts and functional gauges rely on, at no extra inspection cost — the size check already implies the form check. Independency removes that guarantee, so mating clearances must be re-verified directly and gauges built to virtual condition no longer imply form acceptability. Opt out when the envelope genuinely is not wanted; do not opt out for the sake of it.