The Minkowski inner product is not an inner product, since it is not positive-definite, i.e. the quadratic form η (v, v) need not be positive for nonzero v. The positive-definite condition has been replaced by the weaker condition of non-degeneracy. The bilinear form is said to be indefinite. The Minkowski metric η is the metric tensor of Minkowski space. It is a pseudo-Euclidean metric, or more generally, a constant pseudo-Riemannian metric in Cartesian coordinates. As such, it is a nondegenerate symmetric bilinear form, a type (0, 2) tensor. It accepts two arguments u p, v p, vectors in T p M, p ∈ M, the tangent space at p in M. Due to the above-mentioned canonical identification of T p M with M itself, it accepts arguments u, v with both u and v in M.