The nuclear spin energy levels of coupled protons A and X reveal two possible excitation frequencies for A, depending on the spin state of X, and vice versa. The mutual nature of coupling implies that the difference between frequencies A1 and A2 equals the difference between frequencies X1 and X2, denoted by JAX. Because X has approximately equal populations in the spin-up and spin-down states, the numbers of A that are excited by frequencies A1 and A2 are about the same. Likewise, the numbers of X excited by frequencies X1 and X2 are about the same. Indeed, the NMR signal for proton A is split into two peaks with equal intensity, centered at the chemical shift of A, and likewise for X. The peaks in each doublet are separated by JAX, the coupling constant. Spin–spin coupling usually occurs between NMR-active nuclei that are nonequivalent and separated by three or fewer bonds.