Our original plan for workshop No. 3 was to discuss what are generally referred to as material condition modifiers, namely the encircled letters “M,” “L,” and “S” which sometimes follow tolerance values and sometimes follow datum-feature labels in feature-control frames. The idea was to explain their very different effects as a function of their different locations and as a result, to justify some new names, such as tolerance-zone size modifiers when associated with tolerance values, and tolerance-zone mobility modifiers when associated with datum features. Because of many requests to precede this investigation with a preparatory overview of the most important fundamental concepts of GD&T, we do just that, convinced it will enhance the tolerance-zone size and mobility modifier workshop, which will come next.

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