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When datum references are repeated in the second segment of a composite or single segment control, the meaning is different.
The tolerance in the upper segment of a composite tolerance is located by all applicable basic dimensions. On this drawing, the yellow tolerance is located by the yellow dimensions.
The lower segment of a composite tolerance does not use the basic dimensions which originate at the datums. Only the basic dimensions within the pattern are applicable. If a datum is repeated, it indicates that the orientation of the pattern must be held to the tighter tolerance. In this case, the perpendicularity to datum A must be within 0.2 and the pattern of two holes may not tilt more than 0.2 relative to datum B.
In the drawing below you will notice that there are two separate position callouts being used. These single segment callouts are not composite. The upper segment has the same meaning as the upper segment of the composite callout shown earlier. The lower segment, however, improves the location as well as the orientationof the feature(s) relative to the datums referenced in the second sement. Notice that the 19mm dimension from datum B is now also shown in blue. The pattern must be positioned at the 19mm dimension from B within 0.2 total even though the pattern may be out of position as much as 0.6 total relative to datum C.
This tip is in accordance with ASME Y14.5M-2994, ASME Y14.5-2009 and ASME Y14.5-2018.
This tip was originally released in December 1998.