The decay of RF-induced NMR spin polarization is characterized in terms of two separate processes, each with their own time constants. One process, called T, is responsible for the loss of resonance intensity following pulse excitation. The other process, called T, characterizes the width or broadness of resonances. Stated more formally, T is the time constant for the physical processes responsible for the relaxation of the  components of the nuclear spin magnetization vector M parallel to the external magnetic field, B (which is conventionally designated as the z-axis). T relaxation affects the coherent components of M perpendicular to B. In conventional NMR spectroscopy, T limits the pulse repetition rate and affects the overall time an NMR spectrum can be acquired. Values of T range from milliseconds to several seconds, depending on the size of the molecule, the viscosity of the solution, the temperature of the sample, and the possible presence of paramagnetic species (e.g., O or metal ions).