A Zettel is a slip of paper; a Zettelkasten is a “slip box”. Since that phrase doesn’t mean much in English, the German word is often used in English among #Zettelkasten method adherents. Niklas Luhmann’s Zettelkasten resembled a pre-digital-age library card catalog. The actual slips were roughly the size of index cards but on thinner stock. Modern zettelizers most often use digital files in a text management software application.