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Symmetry is essentially the flat version of concentricity, and establishes a TZ composed of two parallel planes, in which the median datum points of non-cylindrical opposing features must fall. This measurement can be done for slots and cutouts, as well as tabs or parallel surfaces. The TZ will again look similar to flatness since the TZ is between two parallel planes. However, the key difference is that the symmetry is in relation to a datum plane. Just as with concentricity, symmetry is a control that is difficult to measure, requiring the calculations of as many midpoints is as realistic for a given cross section, and then again for as many cross sections as is realistic. These median points then need to be compared to a derived datum. This process is typically only practical to do with a Coordinate Measuring Machine, or CMM, which is a high precision 3D scanner. Symmetry, which controls only median points, allows for typically undesired results. For instance a slot drawn like this and controll
