Read the Feature Control Frame to Determine the Origin of Measurement. 
Here is a little test that one of our faithful readers asked me to create. He tells me that 9 out of 10 folks at his work get it wrong. Given the drawing, does figure A or B more correctly illustrate the datum reference framework which establishes an origin of measurement for the profile tolerance and the position callout on the two holes?
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If you said Fig. B, you are correct. The dimensioning may have thrown you off. You cannot go by the dimensioning like we used to in the old plus/minus days. You have to follow the datum precedence stated in the feature control frames. Because the large hole, datum feature C is the third datum referenced, it can only establish the measurement origin from left to right. The up/down origin is established by datum feature B. Since the dimensions are basic, they may be placed on the drawing any number of ways without changing the meaning. In fact, with the new ASME Y14.41 standard, dimensions often will not appear on the drawing.
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