Professor Hendrik Anthony Kramers {Proc.Acad.Sci.Amsterdam, 1930, 2, 432.} has given us the names 'Kramers doublets', 'Kramers degeneracy' and 'Kramers centers'. He showed that a system containing an odd number of electrons, so S = 1/2, 3/2, 5/2 etc., even with no symmetry, and with no magnetic field, must be at least doubly degenerate. A non-Kramers center will have S = 1, 2, 3, etc. A corollory of this is that systems with odd numbers of electrons will relax differently from one with even numbers of electrons.

Kramers theorem is a keystone to EPR. Equally, non-Kramers EPR spectroscopy, has been very important in underestanding the EPR of ferrodoxins such as Fe₃S₄, Fe₄S₄, and Fe₈S₈, and can be observed for haem proteins, iron-oxo clusters, and inorganic model complexes of these.