 A family of degree 0 line bundles parametrized by a "k"-variety "T" is defined to be a line bundle "L" on 
"A"×"T" such that 

Then there is a variety "A"v and a line bundle formula_2, called the Poincaré bundle, which is a family of degree 0 line bundles parametrized by "A"v in the sense of the above definition.Moreover, this family is universal, that is, to any family "L" parametrized by "T" is associated a unique morphism "f"