Symmetry and the Lattice Metric

There exist an infinite but enumerable number of point groups formed by the operations A and l , l n oo. They represent the symmetries of macro- 

2-fold symbol 2 normal to the plane of projection parallel to the plane of projection  

The d planes may exist in orthorhombic F, tetragonal I, cubic I and cubic F Bravais lattices (Section 2.6.1). In the tetragonal, trigonal, hexagonal and cubic systems (Sections 2.5.8 and 2.5.9) we find mirror planes which are not parallel to the (100), (010) or (001) planes. The glides for the n and d planes corresponding to these orientations are oblique with respect to the a, b, c axes. More detailed information may be found in the International Tables for Crystallography. 

In a periodic structure we find series of symmetry elements, i.e. series of rotation and rotoinversion axes as shown in Fig. 2.9. 

---