dtmbiz,
You are right, I should have been more precise.
(Actually I use the ISO-standard not ASME. In ISO tolerances of position, symmetry and coaxiality all apply to the extracted axis or median face of the actual feature.)
Thank's for your valuable comments.
I agree that Figure A is more clear and that the theoretical exact dimensions 30 are consistent with datum B. However, Figure B is more clean in my view and I wish it was more clearly stated in the standard that symmetry is implied in this case.
A related...
Often I have patterns of holes or other features symmetrically located on a part. My question is if the symmetry is implied according figure B in the attached picture or is is neccessary to dimension each of the holes separately to the symmetry plane as in figure A?
