In rotational-vibrational and electronic spectroscopy of diatomic molecules, Hund's coupling cases are idealized descriptions of rotational states in which specific terms in the molecular Hamiltonian and involving couplings between angular momenta are assumed to dominate over all other terms.  There are five cases, proposed by Friedrich Hund in 1926-27 and traditionally denoted by the letters (a) through (e).  Most diatomic molecules are somewhere between the idealized cases (a) and (b).