Of 32 crystal classes (based on 32 point groups, the possible combinations of symmetry elements intersecting at a point) assigned to 7 crystal systems, only 11 are found in common minerals. Each system may be described in terms of three noncoplanar vectors (crystallographic axes) that are generally nonorthogonal as well. Although mineral assignment to a crystal system may require only examination of external crystal morphology, assignment of crystal class commonly requires X-ray diffraction analysis. 609 The crystal systems are triclinic, monoclinic, orthorhombic, tetragonal, trigonal, hexagonal, and isometric.