A molecule that has a centre of inversion, i.e. a molecule with a point taken to be the origin (0,0,0) such that the molecule looks exactly the same under the transformation (x,y,z) → (–x,–y,–z). Thus, a centrosymmetric molecule has inversion symmetry. Examples of centrosymmetric molecules include homonuclear diatomic molecules, square planar molecules ML₄, and octahedral molecules ML₆. Examples of molecules that are not centrosymmetric include heteronuclear diatomic molecules and tetrahedral molecules. An important feature of centrosymmetric molecules that can be proved using group theory is that the same vibrational transition cannot appear in both the infrared spectrum and the Raman spectrum (See Raman effect). This result is known as the exclusion rule for molecular spectra. See also gerade; ungerade.