I read about establishing datum for a width feature of size in "ASME Y14.5-2009" and "GD&T mechanical design".
But there are seem two different way to establish datum, for example in this picture below:
According to "GD&T mechanical design", datum B is the 7 inch size feature between the left and right edges.
But according to "ASME Y14.5-2009", it will be the center plane between left and right edges.
So what is the right datum here? And how to make a mating gage for such center plane