Two common mechanisms by which electrons and nuclei interact are the Fermi contact interaction and by dipolar interaction. The former applies largely to the case of isotropic interactions (independent of sample orientation in a magnetic field) and the latter to the case of anisotropic interactions (spectra dependent on sample orientation in a magnetic field). Spin polarization is a third mechanism for interactions between an unpaired electron and a nuclear spin, being especially important for formula_60-electron organic radicals, such as the benzene radical anion. The symbols ""a"" or ""A"" are used for isotropic hyperfine coupling constants, while ""B"" is usually employed for anisotropic hyperfine coupling constants.