Rotatory-inversion axes

Orthorhombic Three mutually perpendicular 2-fold rotation or rotatory-inversion axes along a, b and c mmm arfib rfi e a = (3 = y = 90 ... 

Improper symmetry operation A symmetry operation that converts a right-handed object into a left-handed object. Such operations include mirror planes, centers of symmetry, and rotatory-inversion axes. 

Point group A group of symmetry operations that leave unmoved at least one point within the object to which they apply. Symmetry elements include simple rotation and rotatory-inversion axes the latter include the center of symmetry and the mirror plane. Since one point remains invariant, all rotation axes must pass through this point and all mirror planes must contain it. A point group is used to describe isolated objects, such as single molecules or real crystals. 

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