This symbol, 2/m 2/m 2/m, indicates the Point Symmetry of the crystal (See the Essay on Crystals), i.e. the set of possible operations that can be applied to the crystal (geometrical operations like rotation and the like) which, each for themselves, leave at least one point unchanged (i.e. it leaves it at the same position) -- thus comprising the so called non-translative movements -- and map the crystal onto itself. For example if we subject the outer form of a human to a mirror-reflection-operation with respect to a certain plane (thought) through the middle of his body, then this form will be mapped onto itself after the execution of this symmetry-operation, and so reappear unchanged. See also the next note.
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