Although a space group is uniquely defined, i.e. the symmetry
operations defining it are uniquely given, the choice of
vectors that defines a unit cell for that symmetry is not
unique. The different choices are known as settings. Most
space groups have only one setting, but it is possible to find
space groups with several settings. For example, "C 1 2/c 1"
has 18 settings. The xHM symbol for setting 1 is
"C 1 2/c 1", the symbol for setting 2 is "A 1 2/n 1", etc.