In general, the "g" factor is not a number but a second-rank tensor represented by 9 numbers arranged in a 3×3 matrix. The principal axes of this tensor are determined by the local fields, for example, by the local atomic arrangement around the unpaired spin in a solid or in a molecule. Choosing an appropriate coordinate system (say, "x","y","z") allows one to "diagonalize" this tensor, thereby reducing the maximal number of its components from 9 to 3: "gxx", "gyy" and "gzz". For a single spin experiencing only Zeeman interaction with an external magnetic field, the position of the EPR resonance is given by the expression "gxxBx" + "gyyBy" + "gzzBz". Here "Bx", "By" and "Bz" are the components of the magnetic field vector in the coordinate system ("x","y","z"); their magnitudes change as the field is rotated, so does the frequency of the resonance. For a large ensemble of randomly oriented spins, the EPR spectrum consists of three peaks of characteristic shape at frequencies "gxxB"0, "gyyB"0 and "gzzB"0: the low-frequency peak is positive in first-derivative spectra, the high-frequency peak is negative, and the central peak is bipolar. Such situations are commonly observed in powders, and the spectra are therefore called "powder-pattern spectra". In crystals, the number of EPR lines is determined by the number of crystallographically equivalent orientations of the EPR spin (called "EPR center").