Fitting into this context is one piece of existing NMR knowledge that makes things more complicated -yet consistent: the early NMR literature had already recognized the phenomenon of observed peak splitting that cannot be explained by any existing or perceivable coupling in the molecule through the first-order n+1 rule. In these not too infrequent cases, the interaction between nuclei can lead to transitions and, subsequently, appearance of additional lines (peak splitting) as a result of mixed spin states for which no pair of nuclei alone can be identified as coupling partners. Coincidentally, the splitting of the signal pattern in these cases is deceptively similar to what would be observed for actual ("real") J-couplings. This was likely the rationale for the introduction of the historic term, "virtual coupling", which has led to substantial confusion as coupling is not the cause of this type of additional peak splitting. The explanation of this phenomenon is, again, rooted in the QM relationships of the underlying nuclei, particularly the relationship between the resonance frequencies and the magnitude of the J-coupling constant.