International
Tables for Crystallography Volume A Spacegroup symmetry Edited by M. I. Aroyo © International Union of Crystallography 2016 
International Tables for Crystallography (2016). Vol. A, ch. 2.1, p. 148

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Notes on the `heights' h of symmetry points , , and : (1) Centres of symmetry and , as well as inversion points and on and axes parallel to [001], occur in pairs at `heights' h and . In the spacegroup diagrams, only one fraction h is given, e.g. stands for and . No fraction means and . In cubic space groups, however, because of their complexity, both fractions are given for vertical axes, including and . (2) Symmetries and contain vertical and axes; their and inversion points coincide with the centres of symmetry. This is not indicated in the spacegroup diagrams. (3) Symmetries and also contain vertical and axes, but their and inversion points alternate with the centres of symmetry; i.e. points at h and interleave with or points at and . In the tetragonal and hexagonal spacegroup diagrams, only one fraction for and one for or is given. In the cubic diagrams, all four fractions are listed for ; e.g. (223): : ; : . 