Suppose that a ferromagnet is single-domain in the strictest sense: the magnetization is uniform and rotates in unison. If the magnetic moment is  and the volume of the particle is , the magnetization is , where  is the saturation magnetization and  are direction cosines (components of a unit vector) so . The energy associated with magnetic anisotropy can depend on the direction cosines in various ways, the most common of which are discussed below.