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If **h** **R** = **h** and **h** **t**
0 (modulo 1) then the reflection *h* is extinct, or absent, i.e. its
amplitude is 0. If we have a 2-fold symmetry, like in *P*2_{1}, the two
contributions to the structure factor are equally large but have exactly
opposite directions and they cancel each other. This is illustrated in Fig. 4a.
In space groups with 3-fold symmetry, such as *P*3_{1}, there will be systematic
absences for reflections where **hR** = **h** and **h** **t** = or , as illustrated in Fig. 4. Because
of the 3-fold symmetry the atoms are divided into 3 groups rather than 2 as was
the case with a 2-fold symmetry. With 4- or 6-fold symmetry the situation is
much like that of the 3-fold, only we now have 4 or 6 contributions, each unlike
in size but differing by 90 and 60 respectively. See Figs.
4c and 4d.

**Copyright © 1981, 1998 International Union of
Crystallography**