The 32 crystallographic point groups
may be combined with the 14 Bravais lattices
to give 230 possible space groups
. These are used in crystallography
to describe the arrangement of symmetry elements
in a crystal
Using X-ray diffraction the space group of a crystalline substance may be deduced from systematic absences in the diffraction pattern. These occur when destructive interference occurs between waves diffracted from different Miller planes, and the forbidden reflection will not be present.