GD&T concept · Rule

Envelope Requirement (Ⓔ) — ISO Only

ISO's opt-in envelope: Ⓔ restores the MMC boundary that ISO leaves off by default.

Definition

The envelope requirement (Ⓔ) is the ISO symbol that requires a feature of size to stay within its envelope of perfect form at MMC — the same behavior ASME gets by default from Rule #1. Under ISO 8015 and ISO 14405-1 that behavior is not the default: ISO specifies independence by default, so Ⓔ must be explicitly written when the envelope is wanted. The symbol is placed in the tolerance compartment, after the tolerance value it qualifies.

The default inversion

The two standards are exact mirror images, and this is the most confused cross-standard point in GD&T. ASME: the envelope is in force unless Ⓘ is shown. ISO: independence is in force unless Ⓔ is shown. The same drawing intent is expressed by different defaults in each system, and a bare size tolerance means opposite things: under ASME it carries an envelope, under ISO it does not. Inspecting an ASME drawing to ISO defaults silently drops the envelope requirement — parts with wavy but in-limit sizes pass when they should fail, or the reverse when the standards are mixed the other way.

What Ⓔ requires

A size tolerance with Ⓔ — Ø10 ±0.1 Ⓔ — carries two requirements at once: the feature's size must stay within limits, and its surface must not extend beyond the envelope of perfect form at the MMC size. Form error is bounded exactly the way Rule #1 bounds it under ASME: none at MMC, growing as the size departs toward LMC.

The dashed boundary is the envelope of perfect form at MMC that Ⓔ restores. Without Ⓔ the ISO default is independence — size and form vary freely; with Ⓔ the feature must stay inside the envelope, the same behavior Rule #1 gives ASME drawings by default. Schematic, exaggerated.

Working across standards

Know which standard the drawing is made to — the title block usually states it; never guess between ASME and ISO defaults
On ASME drawings, do not write Ⓔ — it is not an ASME symbol; the envelope is already the default and Ⓘ is the only opt-out
On ISO drawings, write Ⓔ only where the envelope is genuinely required — otherwise you pay for form control the function does not need
In mixed supply chains, state the governing standard explicitly in the contract or the drawing header; the default difference is real and changes what passes inspection
When a bare size tolerance arrives from a shop using the other standard, ask — the envelope question is a specification question, not an inspection judgment call