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This chapter discusses the spin label theory and spin label structure. The essential structure of the type of spin label is a nitroxide group adjacent to two tertiary carbon atoms. Such a molecule is host to an unpaired electron, and it is a stable free radical and therefore paramagnetic. The disposition of the 2p π orbital of the nitrogen atom, with which the unpaired electron is predominantly associated, and the conventional manner of assigning Cartesian coordinates to the molecule. As a simplification, the N, O, and two tertiary C atoms are placed in a single plane in the diagram although this is not usually strictly correct, the CNO angle being a function of the particular nitroxide molecule considered. It is found that because of the anisotropic disposition of the unpaired electron, its interaction with the large external applied magnetic field is governed by the orientation of the spin-label group with respect …