Example: Let formula_67 be commutative rings, and let "I" be the annihilator of the quotient "B"-module "A"/"B" (which is an ideal of "A"). Then canonical maps formula_68 form a fiber square with formula_69  Let formula_70 be an endomorphism between finitely generated "R"-modules for a commutative ring "R". Then  See also: Herbrand quotient (which can be defined for any endomorphism with some finiteness conditions.) An additive relation formula_35 from a module "M" to a module "N" is a submodule of formula_74 In other words, it is a "many-valued" homomorphism defined on some submodule of "M". The inverse formula_75 of "f" is the submodule formula_76. Any additive relation "f" determines a homomorphism from a submodule of "M" to a quotient of "N" where formula_78 consists of all elements "x" in "M" such that ("x", "y") belongs to "f" for some "y" in "N". A transgression that arises from a spectral sequence is an example of an additive relation.