An -linear "derivation" on is an -module homomorphism formula_2 to an -module with the image of in its kernel, satisfying the Leibniz rule formula_3. The module of Kähler differentials is defined as the -module formula_1 for which there is a universal derivation formula_5. As with other universal properties, this means that is the "best possible" derivation in the sense that any other derivation may be obtained from it by composition with an -module homomorphism. In other words, the composition with provides, for every , an -module isomorphism