Eigenvalues of the Hamiltonian of a polyatomic system define PESs as functions of normal modes Q i of the system (i.e. linear combinations of the nuclear displacements with specific symmetry properties). At the reference point of high symmetry, where the symmetry-induced degeneracy occurs, several of the eigenvalues coincide. By a detailed and laborious analysis, Jahn and Teller showed that – excepting linear molecules – there are always first-order terms in an expansion of the matrix elements of the Hamiltonian in terms of symmetry-lowering (in the language of group theory : non-totally symmetric) normal modes. These linear terms represent forces that distort the system along these coordinates and lift the degeneracy. The point of degeneracy can thus not be stationary, and the system distorts toward a stationary point of lower symmetry where stability can be attained.