and HA[ 13 C b ], respectively. The strong coupling situation is nearly fully lifted, with cleanly separated HA and HB multiplets and near-first order multiplet structures, except for a remaining modest roof effect. Importantly, the observed A-X and B-X splittings on these 13 C satellites are now identical to the actual JAX and JBX couplings within 0.05 Hz precision. This illustrates that 1 H-1 H coupling measurements in many cases should benefit from focusing on 13 C satellite signals. Figure shows a real demonstration on pyridine. In the all-12 C isotopomer, the protons H3 and H5 are chemically equivalent (Dn !", = 0), and since a small 4 JH3H5 coupling exists (1.4 Hz), they are infinitely strongly coupled. Because the couplings from any other proton to H3 and H5 are different, this represents a textbook example of magnetic inequivalence, yielding a second order multiplet (Figure ). In the 13 C3 isotopomer, this chemical shift degeneracy is broken, with