The right figure illustrates the energy diagram of the simplest spin system where a is the isotropic hyperfine coupling constant in hertz (Hz). This diagram indicates the electron Zeeman, nuclear Zeeman and hyperfine splittings. In a steady state ENDOR experiment, an EPR transition (A, D), called the observer, is partly saturated by microwave radiation of amplitude B 1 while a driving radio frequency (rf) field of amplitude B 2 , called the pump, induces nuclear transitions. Transitions happen at frequencies ν ν 1 and ν ν 2 and obey the NMR selection rules Δ Δ M I = ± ± 1 and Δ Δ M S = 0 . It is these NMR transitions that are detected by ENDOR via the intensity changes to the simultaneously irradiated EPR transition. Both the hyperfine coupling constant (a) and the nuclear Larmor frequencies ( ν ν n ) are determined when using the ENDOR method.