Correct and incorrect loading are associated with symmetry and asymmetry in the part configuration With reference to Fig. 7-1 ,planes of symmetry (or asymmetry) are denoted AA,BB,and CC; the corresponding perpendicular axes (sometimes, but not necessarily always,axes of rotational symmetry) are denoted X,Y, and Z . A completely symmetrical part; that is, a part containing three planes of symmetry, can be loaded in a fixture in four different orientations. From an initial position it can be turned 180 degrees around the three axes X,Y, and ,Z, respectively. In other words, it can be turned end-for-end and upside down, and there are no orientations other than these four. Apart from any surface markings there is no discernible difference between the four positions ,and any machined configuration applied to the part will produce the same end result. Every position is a correct position and incorrect machining is simply not possible in this case, regardless of how the part was loaded.