  We introduce a reducibility on classes of structures, essentially a uniform
enumeration reducibility. This reducibility is inspired by the Friedman-Stanley
paper on using Borel reductions to compare classes of countable structures.
  This reducibility is calibrated by comparing several classes of structures.
The class of cyclic graphs and the class of finite prime fields are equivalent,
and are properly below the class of arbitrary finite graphs. The class of
finite graphs and the class of finite linear orders are maximal among all
classes of finite structures.
  We also prove some general characterizations of reducibility to certain
classes. Examples of large chains and antichains of classes are constructed.
