In a cochain complex of modules ...->C^(i - 1) ->^(d^(i - 1)) C^i ->^(d^i) C^(i + 1)->..., the module B^i of i-coboundaries is the image of d^(i - 1). It is a submodule of C^i and is contained in the module of i-cocycles Z^i. The cochain complex is called exact at C^i if B^i = Z^i.