The zettelkasten ("slip-box" in German) is a note-taking and organisational system credited to German sociologist Niklas Luhmann, who both coined the term and made a notably extensive use of it in the development of his research and publications. The basic premise has undergone considerable evolution since its earliest recognisable form in the 1500s, when Conrad Gessner indexed the contents of his notes into his commonplace book by cutting and gluing relevant slips of notes into the book under subject headings. Thomas Harrison, an English inventor, devised the first recognisable index card catalog in the 1640s, the precursor to library catalogs and Luhmann's own system, which works in this manner:

First, a blank index card is selected, onto which notes on a specific subject are taken, with as much granularity as possible, aiming to fit precisely one idea on every index card. Primary source citations are also indexed on this card, typically on the back, for ease of bibliographic citation later. Every time this idea is re-encountered from a new source, that source is added to this bibliography, unifying that idea across many sources.

Second, every index card is assigned an "address," a combination of letters, numbers, or other symbols which identifies that card and the ideas on it unambiguously.

Third, when another card gives reference to the ideas on this card, it is annotated with the "address" of this card, and this card is given a mutual annotation: a primitive bidirectional hyperlink.

Fourth, when the idea present on this initial card is to be extended with further ideas, new cards are made for those ideas, which use addresses derived from the address of the parent index card. For example, if the parent card was 1A2B3C, three resulting branched ideas might be given the addresses 1A2B3C1,1A2B3C2, and 1A2B3C3. The Johnny Decimal System developed in 2021 by John Noble is a variant of this method which reduces it to strictly numerical addresses in a fixed set of ten main topical categories.

Fifth, though not treated as necessary in all cases, there might be a master index made for the purpose of locating specific cards and their ideas, giving general subject headings, similar to the properties of the Dewey Decimal System.

Compared to other note-taking methodologies, which might focus on prioritising skill gain, knowledge retention in order to pass an exam, or preparation for public speaking and debate, the goals of zettelkasten tend to be structured around a future intention of publication. The maintaining of single ideas with their full bibliographic citation readily to hand, acting as nodes between other related ideas and also between many sources which agree on the same idea, is highly effective for coordinating those ideas into a clear piece of writing with good academic rigour. It clearly worked for Luhmann, whose 90,000 index cards spawned over 400 scholarly articles and more than 70 books between 1963 and 2006.

Zettelkasten has become an increasingly popular system with the millennial rise in interest in Getting Things Done, Evergreen Notes, lifelogging, and other productivity and documentation methods. The advent of digital note-taking software such as Amplenote and Roam, which feature bidirectional linking, have made it even more expedient, by decoupling it from physical paper media, and granting it the ease of digital copying, pasting, and screen capture.

Iron Noder 2022, 14/30