The sociologist Niklas Luhmann kept a card index from the early 1950s into the 1990s. It grew to about 90,000 slips of paper. He described it as a partner in communication. Meine Produktivität ist im wesentlichen aus dem Zettelkasten-System zu erklären, he said: “My productivity can essentially be explained by the card index system.” Bielefeld University acquired his papers in 2011, and in 2019 the card index went online.