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When GD&T is used, the geometric tolerances apply to the features - not the dimensions. Therefore when BASIC dimensions are used with geometric tolerances, several different dimensioning schemes may be used without changing the meaning of the drawing. The first drawing below is an example of baseline dimensioning. The other two illustrate chain dimensioning. Since the position tolerance is related to datums A, B and C, all three drawings have the same meaning even though the dimensioning is different. This would not be true if the dimensions locating the holes were toleranced rather than BASIC. Because the meaning of these drawings is the same, the designer should consider the needs of those who will read the drawing when placing dimensions.
This tip is in accordance with ASME Y14.5M-1994, ASME Y14.5-2009 and ASME Y14.5-2018.
This tip was originally released in April 1999.