Any crystal which has a mirror plane as one of its symmetry elements has the property that its mirror image (with any plane as the mirror plane) can always be superimposed on the original crystal by translation or rotation or both. If there are no mirror planes as symmetry elements then the mirror image of a crystal cannot be brought into superposition with the original crystal by rotation or translation. This is enantiomorphism, and the mirror images are said to be enantiomorphs of each other. The possibility of enantiomorphic crystals is determined by the crystal symmetry, i.e., by the point group of the crystal species. There are 32 possible point groups, and 22 of these are capable of forming enantiomorphs. The enantiomorphs are designated right or left handed, according to whether they rotate the plane of polarised light to the right or to the left. Sometimes it is clear from the outward form of the crystal whether it is right or left handed, and sometimes optical methods are needed to determine this.