Cauchy's stress principle asserts[1]:p.47–102 that as Δ S {\displaystyle \Delta S} becomes very small and tends to zero the ratio Δ F / Δ S {\displaystyle \Delta \mathbf {F} /\Delta S} becomes d F / d S {\displaystyle d\mathbf {F} /dS} and the couple stress vector Δ M {\displaystyle \Delta \mathbf {M} } vanishes. In specific fields of continuum mechanics the couple stress is assumed not to vanish; however, classical branches of continuum mechanics address non-polar materials which do not consider couple stresses and body moments.