The discussion about the dimensions of the theatre needed a scientific base for the imagination of the Roman Theatres original historic appearance. Fortunately, Stefan Schmitz, a private sponsor of cultural projects in Mainz and Rheinland-Pfalz ordered a scientific research for a serious study on the historic building conditions of the theatre. The results, in form of graphic accounts of a theoretical reconstruction, should extend the state of knowledge of the involved participants of planners and decision makers in policy and administration and of course fuel the imagination of the public discussion. With extensive investigation and verification of the excavation documents and by examination of the original construction conditions of the Roman remaining building elements the most appropriate  construction principles could  be defined as for the gradient ratio of the Cavea stairs above construction domes, the position of the Praecinctio and the access possibilities from inside and outside. Because detailed architectural design elements for columns or the outside façade could not be verified by the excavation, abstract theoretical reconstruction elements are applied in plausible dimensions.