There are volumes of different nature than the gradients ( vectors ) that can be and are the source of directional information . These are the size of the tensor , such as a mechanical tension (s ) or growth rate (w). Tensor stands in the three orthogonal directions , which assume extreme values ​​. These are the main directions of the tensor (PD - esses ) . While the vector has direction and sense , PD has only direction . The most important difference between vector and tensor , however, concerns the symmetry ; tensor is characterized by mirror symmetry .