Fortunately, we have outstanding Ground Truth data: the Gesamtkatalog der Wiegendrucke (GW) (Staatsbibliothek zu Berlin 2019a) and its side project, the Typenrepertorium der Wiegendrucke (TW) (Staatsbibliothek zu Berlin 2019b). Both were initiated by Konrad Haebler at the turn of the last century and are still maintained today at the Berlin State Library. The GW provides us with bibliographical data for all known incunabula editions (books printed in the 15th century) as well as some 15,000 digital copies from all over the world. The corresponding records in the TW list over 6,000 fonts used for these books and later editions. This Ground Truth data was painstakingly collected in over a century and was recently - thankfully - converted to a database by the Berlin State Library (Eisermann / Duntze 2014).