Envelope Principle (Rule #1)

Under the envelope principle, size tolerance implicitly controls form. A shaft with diameter 10.0 +/- 0.1mm must fit within a perfect cylinder of 10.1mm diameter (MMC for external feature). Even if the shaft measures 9.95mm at one cross-section (within size tolerance), it cannot bow or bend enough that any point exceeds the 10.1mm envelope.

This is a default rule in ASME Y14.5 but NOT in ISO. On ISO drawings, the envelope principle only applies if explicitly invoked (circled E symbol or ISO 8015 reference). Without it, size tolerance controls size only, and form must be controlled separately by ISO 2768-2 geometrical tolerances or explicit GD&T.

This difference directly affects CMM evaluation. On ASME drawings, the CMM checks both size and form simultaneously. On ISO drawings (without the envelope principle), size and form are evaluated independently.