The dimension origin symbol is used in Geometric Dimensioning and Tolerancing to indicate where a dimension must be measured from. This symbol is used in place of a dimensional arrow leader to indicate the origin for measurement. An example of using the dimension origin symbol is shown below in Figure 1.
Figure 1: Dimension Origin Symbol
Using the dimension origin symbol is very similar to creating a datum surface for your part to measure size from. When measuring a dimension that contains the dimension origin symbol, you must measure from the identified surface as the origin plane. You are not allowed to flip the part over and measure from the other direction. Figure 2 shows the correct (and incorrect) way to measure the dimension shown for the part in Figure 1.
Figure 2: Correctly Measuring when Dimension Origin Symbol is Used
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