NMR-active nuclei have energy levels called 'spin states' that are associated with the orientations of their nuclear magnetic moments. In the absence of a magnetic field, the nuclear magnetic moments are randomly oriented, and the spin states are degenerate. When an external magnetic field is applied, the spin states have only 2 + 1 orientations available to them. A proton with = ½ has two available orientations. Similarly, for a quadrupolar nucleus with a nuclear spin value of one, the nuclear magnetic moment has three possible orientations.

The orientations aligned against the field are higher energy than those aligned with the field. The quantization of spin states is a phenomenon called Zeeman splitting. The energy difference between the two spin states is proportional to the strength of the applied magnetic field. However, at temperatures close to absolute zero, the lower energy spin state is more populated than the high energy spin state.