The relation between plastic deformation of a crystal and mechanical impact is unambiguously established by means of symmetry.The shape of impact or pressure is the deformation at a point. The symmetry of a resting point, according to A. V. Shubnikov, is equal to ∞/∞ m, and the symmetry of a point moving in one direction is equal to ∞ m. The polar vector possesses such symmetry. The stress on the face produced by this vector can be equivalently described by a two-dimensional polar tensor with symmetry ∞ m.