 Over fields of characteristic not 2 they are quotients of K3 surfaces by a group of order 2 acting without fixed points and their theory is similar to that of algebraic K3 surfaces.Enriques surfaces were first studied in detail by as an answer to a question discussed by about whether a surface with "q"="p""g" = 0 is necessarily rational, though some of the Reye congruences introduced earlier by are also examples of Enriques surfaces.Enriques surfaces can also be defined over other fields.