Rule #1 — The Envelope Principle

The envelope principle: perfect form at MMC, with form error bounded as the size departs from MMC.

Definition

Rule #1 — the envelope principle — is the ASME Y14.5 default that ties a feature of size's form to its size: the surface must not extend beyond the boundary (envelope) of perfect form at MMC. A pin at its largest allowed size must be perfectly straight, round, and cylindrical — there is no room left for form error inside its own boundary. As the produced size departs from MMC toward LMC, form error is allowed to grow, up to the amount of the size departure. The size tolerance therefore carries a built-in form tolerance, and that form tolerance is exactly zero at MMC.

The envelope

The envelope is the boundary of perfect form at the MMC size — the ideal cylinder or width pair the feature would have if it were perfect at its worst size. The rule says the actual surface may bow, taper, or go out of round, but never so far that it breaks through that boundary. A Ø10 ±0.1 pin produced at Ø9.95 may bend by up to 0.05 — the distance from its size back to the Ø10 envelope — and no more. The form error is paid for out of the size departure, one for one.

What Rule #1 controls — and what it doesn't

Rule #1 bounds the form of features of size: cylinders and widths between parallel planes. Straightness, roundness, and cylindricity of such a feature are already limited by size plus Rule #1 — which is why a separate form callout is only added when the function needs tighter form than the size-derived envelope provides, and why such a callout layers on top of the envelope rather than replacing it. Rule #1 does not control flatness of a surface (a flat face is not a feature of size), and it says nothing about orientation or location: a perfectly cylindrical pin can lean, and Rule #1 will not complain.

Rule #1 and MMC are the same boundary

The envelope is exactly the boundary the Ⓜ modifier builds on: the MMC boundary of a position callout is the virtual condition, and a functional gauge checks that boundary. This is why MMC and the envelope belong together — when a feature fits a gauge built to virtual condition, Rule #1 is what made its form acceptable along the way. Axis straightness at MMC is the one Form control that engages this boundary directly: its zone is the cylindrical envelope around the derived median line, measured from the same MMC logic. See the Maximum Material Condition and Straightness pages for those mechanisms.

When Rule #1 does not apply

• When the drawing explicitly invokes independency Ⓘ — the ASME opt-out that removes the envelope (see the Independency page)
• Under ISO drawings, where independence — not the envelope — is the default, and Ⓔ restores envelope behavior (see the Envelope Requirement page)
• For non-rigid parts, which are handled with their own special rules when flexibility is part of the function
• For features that are not features of size — surfaces, and any control that does not involve a size-derived boundary

What it means for your design

• You usually do not need to add straightness or cylindricity to a plain pin or bore — size plus Rule #1 already bounds form
• When the envelope is not what you want — waviness must be tolerated, or form is controlled separately — say so explicitly with Ⓘ
• When you need tighter form than size provides, add a form tolerance: it stacks on top of the envelope instead of replacing it
• For ISO work, remember the default is flipped — without Ⓔ there is no envelope at all