Many holdings of the Main Library are kept in closed stacks. This means that, with the exception of frequently used reference works, the majority of the holdings are are not on the open shelves in the reading rooms. Therefore, you have to request books to be able to use them.
Oversize books (shelf marks III, IV, V) as well as books that are older than 100 years can be used in the reading room for old books (Lesesaal Altes Buch).
If you would like to view a large number of volumes with the same shelf mark (e.g. newspapers) directly in the stacks, you can get a permission to enter the closed stacks by arrangement. Note that this applies only to the stacks in the man building, not to the external depot.