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inversion axis In crystallography, an axis of symmetry which can be inverted through 180° about its centre in order to achieve a higher degree of symmetry for the crystal. Thus, an axis of two-fold symmetry on inversion every 90° becomes an axis of four-fold symmetry. An inversion axis also obviates the need for a centre of symmetry. The notation used to indicate an inversion axis is a ‘bar’ above the maximum number of repeat positions, e.g. 2̄ and 4̄ for a two-fold and four-fold inversion axis respectively.