Because the components of vectors and their duals transform differently under the change of their dual bases, there is a covariant and/or contravariant transformation law that relates the arrays, which represent the tensor with respect to one basis and that with respect to the other one. The numbers of, respectively, (contravariant indices) and dual (covariant indices) in the input and output of a tensor determine the "type" (or "valence") of the tensor, a pair of natural numbers , which determine the precise form of the transformation law. The "" of a tensor is the sum of these two numbers.