The linear relation between formula_9 and formula_10 follows from the fundamental laws of conservation of linear momentum and static equilibrium of forces, and is therefore mathematically exact, for any material and any stress situation. The components of the Cauchy stress tensor at every point in a material satisfy the equilibrium equations (Cauchy’s equations of motion for zero acceleration). Moreover, the principle of conservation of angular momentum implies that the stress tensor is symmetric, that is formula_29, formula_30, and formula_31. Therefore, the stress state of the medium at any point and instant can be specified by only six independent parameters, rather than nine. These may be written