It is the parity-violating emission of a positron preferentially along the instantaneous spin direction when the muon decays, that makes μSR possible. The muon decays independently of its environment, of course; but prior to decay, its spin would have been coupled to unpaired electrons or nearby nuclei and it would have undergone Larmor precession in magnetic fields transverse to the spin direction. Therefore, some measure of the chemical state occupied by the muon during its short lifetime is revealed by its spin vector at the moment of decay.

There are three distinct types of techniques each utilizing the muon's asymmetric decay and its spin polarization, which happen to be covered by the acronym μSR: muon spin rotation, muon spin relaxation, and muon spin resonance. These different methods are illustrated in Figure 3.1. In the first, one measures the rotation of the muon spin in a transverse magnetic field; in the second, one follows the relaxation of the initial spin polarization in a longitudinal magnetic field; and in the third, one observes the polarization through the resonant absorption of microwave power due to transitions between hyperfine substates. Most of the recent chemical studies have been performed using the first – the rotation method – so the major emphasis here will be on it. Basically it is used in three ranges of magnetic fields: 2–15 G (1 gauss = 10−4 tesla, T) for free muonium atoms (MSR); 50–200 G for diamagnetic muon states (μSR); and 500–5000 G for Mu-containing free radicals (MRSR).