Niklas Luhmann (1927-1998) was a German sociologist, philosopher of social science, and prominent thinker in systems theory. His Zettelkasten used simple paper index cards stored in wooden files like this:
Niklas Luhmann's Zettelkasten had approximately 90,000 index cards linked altogether using a special numbering system. Each card had a unique number or ID that identified and connected them to projects, subjects and ideas. His numbering system suggested the idea of structure, and depth, creating dependency between zettels. Zettels could also be categorized with meaningful tags. During his academic life, Niklas Luhmann published 50 books and more than 600 articles using his tool and methodology. With the help of his Zettelkasten, Niklas Luhmann was able to effectively read a tremendous amount of books and scientific papers. Around 18,000 references were found in his Zettelkasten!