For example, AB indicates two nuclei which have similar chemical shifts (Δδ similar to or smaller than J), whereas AX indicates two which lie further apart on the spectrum (Δδ significantly larger than J). A2B would similarly indicate a spin system containing two equivalent nuclei (A) and a third, inequivalent one (B). Nuclei which are in equivalent chemical environments (that is, symmetry-related), but inequivalent magnetic environments are distinguished with a prime; e.g. AA'. This key aspect of the notation, i.e., using a prime to differentiate between chemical equivalence only compared to full magnetic equivalence, was introduced by Richards and Schaefer in 1958.
