Often designers need to create symmetrically shaped parts. Symmetrical parts are usually easier to assemble, look better and help maintain balance in a design. Features shown symmetrical must be controlled to avoid incomplete drawing requirements (2.7.3 of ASME Y14.5M-1994). Symmetry is an option in these situations but it is difficult to measure since it requires deriving the features’ median points to determine if they are contained within the specified tolerance zone which is centered on the datum axis or datum center plane.
Position may also be used to assure a symmetrical relationship. The advantages of using Position include the ability to modify the tolerance and datum reference at RFS (implied in 1994 Standard), MMC or LMC. In addition, verification is usually easier for Position than that required for Symmetry since it is the center plane of the Actual Mating Envelope (simulated by the inspection equipment) that must be within the tolerance zone.