These groups may contain only two-fold axes, mirror planes, and/or an inversion center. These are the crystallographic point groups 1 and  (triclinic crystal system), 2, m, and  (monoclinic), and 222, , and mm2 (orthorhombic). (The short form of  is mmm.) If the symbol contains three positions, then they denote symmetry elements in the x, y, z direction, respectively.