The trial (Hauptverhandlung) is the main and central part of the whole criminal proceeding. All that is brought forward in the trial and only what is brought forward there can furnish the basis for the verdict. Whatever has preceded the trial proper becomes irrelevant as soon as the trial is opened.
The principles governing the trial are publicity, orality, immediateness, and concentration.
Hans J. Wolff,
CRIMINAL JUSTICE IN GERMANY: II,
Mich. L. Rev.
Available at: https://repository.law.umich.edu/mlr/vol43/iss1/6