Space Group Symbols of Triclinic and Monoclinic
There are only two space groups possible in the triclinic crystal system. These are \(P1\) and \(P\bar{1}\). There is little subtlety here.
For monoclinic space groups, normally only one of the three symbols is given in the short space group symbol. In typical settings, the \(b\) axis is the unique axis, though in reality this can be chosen to be a different axis.
The space group \(Pm\) (\(P1m1\)) has primitive centring and a mirror plane perpendicular to the \(b\) axis. The space group \(C\frac{2}{m}\) (\(C1\frac{2}{m}1\)) has C-centring, a two-fold rotation axis parallel to \(b\) and a mirror plane perpendicular to \(b\). It is not particularly difficult to understand triclinic or monoclinic space group symbols.