For symmetric rotors a quantum number "J" is associated with the total angular momentum of the molecule. For a given value of J, there is a 2"J"+1- fold degeneracy with the quantum number, "M" taking the values +"J" ...0 ... -"J". The third quantum number, "K" is associated with rotation about the principal rotation axis of the molecule. In the absence of an external electrical field, the rotational energy of a symmetric top is a function of only J and K and, in the rigid rotor approximation, the energy of each rotational state is given by