Postal voting was established in Germany in 1956. Based on the legal latitude of the national legislator, the Federal Constitutional Court confirmed the constitutionality of postal voting several times. In contrast, the constitutionality of electronic voting machines, which were used for federal elections from 2002 to 2005, was rejected as the possibility to control the essential steps in the election was not provided to all citizens. These two cases emphasize that the legal system allows to limit realization of election principles to the advantage of other election principles, but that there are limits. In order to introduce new voting systems, in particular Internet voting systems, it is essential to have guidelines on what is and what is not acceptable. This work provides such guidelines. It identifies the principles of the legal latitude in the German constitution, and captures this latitude in a model. This model enables a review of the constitutionality of new voting systems.