text: Nuclear spins exhibit a magnetic dipole moment, which generates a magnetic field that interacts with the dipole moments of other nuclei (dipolar coupling). The magnitude of the interaction is dependent on the gyromagnetic ratio of the spin species, the internuclear distance r, and the orientation, with respect to the external magnetic field B, of the vector connecting the two nuclear spins (see figure). The maximum dipolar coupling is given by the dipolar coupling constant d,
  where γ and γ are the gyromagnetic ratios of the nuclei,  is the reduced Planck's constant, and  is the vacuum permeability. In a strong magnetic field, the dipolar coupling depends on the angle θ between the internuclear vector and the external magnetic field B (figure) according to
  D becomes zero for . Consequently, two nuclei with a dipolar coupling vector at an angle of θ = 54.7° to a strong external magnetic field have zero dipolar coupling. θ is called the magic angle. Magic angle spinning is typically used to remove dipolar couplings weaker than the spinning rate.