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Each space group has a unique number associated with it which is independent of the choice of unit cell and the labelling of the axes. The space groups are numbered from 1 to 230 and are ordered according to the 7 crystal systems: triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and cubic.
Within each crystal system, the space groups are ordered by point group (e.g. 2 < m < 2/m) and then lattice type (e.g. P < A,B,C < F < I ). Further ordering is based largely on the symmetry elements present: In general, rotation axes come before screw axes and mirror planes before glide planes.