Back in the 1940, when Stanley Parker, Mr. GD&T, decided it was time to create a set of tolerancing tools that realistically dealt with reality, two objectives were near the top of his list.

The first was to find a way to encode the fact that as bores get larger they may become ever more offset from their nominal locations and still accommodate a mating shaft. His idea was that stating that on the drawing made it possible for manufacturing to take advantage of this gift of Mother Nature, and for quality assurance to base evaluation on real function rather than mere numbers. His second objective was to find a way to encode the play, or slop, which is often present between mating parts at the beginning of an assembly process, and which allows shifting one slightly relative to the other to accommodate minor location errors in related mating features. The objective again, was to make it possible to accept parts based on their functionality and avoid rejecting them based on criteria that fail to take reality into account.

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