Key Points
A feature of size is a hole, pin, slot, width, or sphere defined with a plus/minus tolerance.
Rule #1: The Envelope Principle – The surface of a feature of size shall not extend beyond a boundary of perfect form at MMC. Each local size must also be within the size limits.
Rule #1 requires features of size to have perfect form at MMC. This insures the pin will fit in the hole. (MMC is the smallest hole or biggest pin)
Example Drawing
The min clearance is calculated as MMC hole – MMC pin .875 – .874 = .001
The max clearance is calculated as LMC hole – LMC pin .880 – .870 = .010
How to Read It
There is no default requirement for perfect form at LMC. Thus, a feature produced at its LMC limit is permitted to vary from true form to the maximum variation allowed by the size tolerance.