Slip Boxes for Literature & Permanent notes
Niklas Luhmann had a slip box system, which he had one box for literature notes and one for permanent notes
The idea is to number each note, something like 1, 1a, 1b, 1b1, 1b2, 1c and so on, so that you can connect notes
So when he created an literature note, he got his slip-box and figure it out where he would store that zettel
So if the literature note he wrote is a followup idea of 1b, he would write something like 1b1 and put it behing note 1b