Interest in the JTE increased after its first experimental verification. Various model systems were developed probing the degree of degeneracy and the type of symmetry. These were solved partly analytically and partly numerically to obtain the shape of the pertinent potential energy surfaces (PES) and the energy levels for the nuclear motion on the JT-split PES. These energy levels are not vibrational energy levels in the traditional sense because of the intricate coupling to the electronic motion that occurs, and are better termed vibronic energy levels. The new field of ‘ vibronic coupling ’ or ‘vibronic coupling theory’ was born.