Even when using the correct term, frequency difference, a number of questions remain: Does this frequency difference refer to peaks, lines, or transitions? How precisely does a measured frequency difference reflect the coupling constant? Connected with the latter, and considering that NMR tables throughout the literature are filled with examples where the J value of a pair of coupled hydrogens is listed as having two (slightly) different values: is it possible that a coupling constant is not constant, i.e., differs depending on each nucleus? Can frequency difference for the same J coupling differ between the resonances of coupled nuclei? And if so, how can this be explained? Finally: how can coupling constants be determined from lines that show no evidently useful frequency differences?