International
Tables for Crystallography Volume A Spacegroup symmetry Edited by Th. Hahn © International Union of Crystallography 2006 
International Tables for Crystallography (2006). Vol. A, ch. 1.4, p. 10
Section 1.4.6. Symmetry axes parallel to the plane of projection^{a}Institut für Kristallographie, RheinischWestfälische Technische Hochschule, Aachen, Germany 
^{†}The symbols for horizontal symmetry axes are given outside the unit cell of the spacegroup diagrams. Twofold axes always occur in pairs, at `heights' h and above the plane of projection; here, a fraction h attached to such a symbol indicates two axes with heights h and . No fraction stands for and . The rule of pairwise occurrence, however, is not valid for the horizontal fourfold axes in cubic space groups; here, all heights are given, including and . This applies also to the horizontal axes and the inversion points located on these axes.
