The "length" of a finite injective resolution is the first index "n" such that "I""n" is nonzero and "I""i" = 0 for "i" greater than "n". If a module "M" admits a finite injective resolution, the minimal length among all finite injective resolutions of "M" is called its injective dimension and denoted id("M"). If "M" does not admit a finite injective resolution, then by convention the injective dimension is said to be infinite. As an example, consider a module "M" such that id("M") = 0. In this situation, the exactness of the sequence 0 → "M" → "I"0 → 0 indicates that the arrow in the center is an isomorphism, and hence "M" itself is injective.