‘The Sum Total of All Human Knowledge’, Part VI

Implementing the schema

In this post, we look more closely at how to implement the combined and refined OoK+UDC schema using physical note-cards. These will include both the standard note-bearing cards (zettels), as well as ancillary ‘structure’ cards which are used to organise and ‘situate’ the note-bearing cards within the overall knowledge structure defined by the OoK+UDC schema.

Physical index cards

Recall the quote in Part III from the blogger abramdemski who suggested

I strongly recommend trying out Zettelkasten on actual note-cards, even if you end up implementing it on a computer. There’s something good about the note-card version that I don’t fully understand. [emphasis in original]

This suggestion implied the need to consider carefully what sort of indexing schema to use, given that physical cards can only occupy a single location within the knowledge structure. Because it is not possible to do an electronic search for key terms or tags in a physical system, the form of the knowledge indexing structure is of key importance for usability. After some exploration of options in Part III, we ended up going with the OoK with UDC enhancements.

The advantage of using the ‘relative looseness’ of the OoK as the foundation for the schema is that a very great deal of filing information is already implicit in the classmarks, without the much higher degree of structuring that is found in the UDC. This means that a lot of the cross-indexing which would be required in a more ad hoc or ‘emergent’ structure is unnecessary. The outline itself provides a good deal of location information for the note-bearing cards. This is manifested in the form of what will here be called ‘structure’ cards.

The schema’s 51 ‘bulk structure’ cards

Essentially, in the OoK as published, there are up to 7 levels of nesting (which were described in Part IV). At the top or highest level are the Ten Parts, numbered 0 to 9 in this implementation. Under each of these Parts there are between 2 to 7 Divisions, and in each of these Divisions there are between 1 to 11 Sections. A look at pp.9-15 of the Propædia shows this “synoptic” outline, as Adler (1994, 7) called the Table of Contents. This is (so to speak) the ‘bulk structure’ of the OoK. There are in total 41 Divisions and 177 Sections, before any personalised additions are made. This bulk structure is also what is shown on the Wikpedia page. We can re-create this level of detail using just 51 ‘structure’ (or ‘divider’) cards, 10 of which (the Parts) will need tabs (at least to start with). At the outset, the entire structure will fit into a small card box, but it will very soon outgrow this.

The standard note-bearing cards, the zettels, are always blank (i.e., unlined). They will generally be white, as well. All the divider/structure cards are lined and coloured (see below). This allows the structuring to be seen at a glance sitting within and bringing organisation to the white zettels. Generally, the writing on all cards, both structure and note-bearing, will be in black ink, unless there is a specific reason to use a different colour, of which there are a couple of cases. These cases will be mentioned below as we progress.

The 10 top-level lined divider cards are tabbed, with the Part number written on the tab, and with text taken from the Simple Wikipedia version of Propædia used as a subheading underneath the main heading, listing the Major Subjects in that Part. For example, Part 1 has the number ‘1’ on its tab, and the heading Matter & Energy is written on the top of the lined section of the card. Directly underneath on the next line is the text from the ‘Main Subjects’ column for Part 1 given on the page for Propædia in Simple Wikipedia, namely: (Physics, Chemistry, Astronomy). A blank line follows, and then each Division gets a single arabic numeral specific to that Division along with a brief caption; recall from Part IV that we convert roman numerals to arabic at level 2 (Division) and level 7. Thus, the (top-level) tabbed Part divider has the next level down (Division) listed on its front. For our example, the three Divisions of Part 1 are written:

  • 1 Atoms:
  • 2 Energy, Radiation & States of Matter
  • 3 The Universe:

A caption that also possesses a sub-title (such as ‘Atoms’ and ‘The Universe’ in the above) will have just the trailing colon ‘:’ shown in the listing to indicate that a more complete caption including a subtitle exists, and can be found on the actual card at the next lower level of nesting. A small space is also left at the LHS of the numbering, for reasons soon to be explained below.

Each of these Divisions will now get its own (lined) structure card. The Division cards are red/pink. These have the full caption as their title, possibly split across multiple lines, where needed, and their 2-digit classmark written at the top right. A blank line is left under the caption, then all the Sections contained in that Division are listed on the Division card with their (single) number and a short(ened) caption. A small space is left at the LHS of each of the Section listings for a dot ‘·’ which indicates which of those Sections is actually being used (as opposed to merely being listed). This way, we know what should go there if necessary (via the Section names), but we don’t create the actual Section cards until something actually needs to be filed into a Section (or deeper). This practice of leaving a blank line after the caption and a space on the LHS for an ‘indicative dot’ is repeated for all structure cards at all levels of use which require it (i.e., from 1 to 6), as is the practice of indicating a shortened caption using the ‘trailing colon’.

Continuing our example, the full caption of the Division card for 13 is The Universe: Galaxies, Stars, the Solar System, with The Universe written on the top line, and the sub-title directly underneath it. A blank line follows, as noted above, and then the three Sections which are in this Division are listed with their captions (again, leaving a small space at the LHS):

  • 1 The Cosmos
  • 2 Galaxies & Stars
  • 3 The Solar System

In this way, each successive level of captioning and nesting reveals more of the finer structure of the OoK. A shortened caption is used (if required) on the next-upper-level card (i.e., the Division), and the full caption is used on the actual card for the Section (suitably split across lines if necessary). This practice is also repeated at all levels of nesting/sectioning.

In this way, the ‘bulk structure’ of the entire OoK down to Section level can be seen without having to write up every single Section card, unless one wants to. The 41 red/pink Division cards filed into their 10 respective Parts show the full ‘bulk structure’ of all the 177 Sections of the initial non-personalised form of the OoK which, as noted above, can be found on the full Wikipedia page.

Hereafter we will only need to create Section cards when there is an actual requirement to do so, for example, if there is an actual note-bearing zettel that needs filing. The structure needed to reach the level of filing required is then created—Section, sub-section, sub-sub-section and so on—so as to avoid creating any ‘orphaned’ child sections lacking their respective ‘parent’ section. This way, the detailed structure of the OoK is filled in only for those parts of the OoK which are actually being used for filing. Rather than over-engineer the initial creation of the overall knowledge structure, we instead simply establish an orienting scaffold or skeleton of the knowledge structure, and then fill in only those parts that actually get used. This represents a much more tractable and proportional approach to filling in the details.

Filling in the deeper detailed structure

Moving deeper, Section cards are orange. As for the Division cards, these have their full title and any subtitle split across multiple lines, if needed (rather than any abbreviated caption from the Division card); their 3-digit classmark at top right; and they list the captions (shortened if necessary) together with, in this case, the letters of the sub-sections.

This process now repeats down to level 7, if and when it is necessary to go to such a degree of nesting. The structure card colours are: red/pink (2-digit); orange (3-digit); yellow (4-digit); green (5-digit); blue (6-digit) and purple/lavender/mauve (7-digit), all with their classmarks at top right, a blank line under the full caption, and a small space at the LHS of the listings to leave room for the ‘indicative dot’. There are no sub-sections deeper than level 7 in the OoK, so no deeper listings will appear on these cards, and one is perhaps better off not even attempting to go any further than this.

Now let us see how this is actually done in practice by continuing the example from above.

131 The Cosmos will have the title at the top of the orange Section card, the classmark 131 at top right, a blank line, and the 5 sub-sections contained within that Section listed:

  • A Structure & Properties of the Universe
  • B Gravitation:
  • C Celestial Mechanics →126B
  • D Properties of the Space-time Continuum:
  • E Origin & Development of the Universe

The sub-section 131.C in OoK contains no further information other than a single redirection instruction ‘see 126.B’. This is therefore placed into the sub-section listing on the Section card, as shown, here rendered with a right arrow, thus: ‘→126B’, with both arrow and classmark written in red ink. For clarity, we could also create a yellow 131C card with the title and the red →126B redirection given either in the header close to the 131C classmark, or directly underneath it, and the card otherwise left blank. In general, this is a good practice to adopt anywhere there is a ‘see’ redirection given on a structure card.

The sub-section 131B will have a yellow card with the term ‘Gravitation:’ at the top, the classmark 131B at top right, and the (longish) subtitle found after the colon in the caption from OoK written on the two lines underneath: ‘a universal force of mutual attraction that is postulated as acting between all matter’. Then the five sub-sub-sections are listed, following a blank line, and again leaving a small space at the left hand side:

  • 1 Development of gravitational theory
  • 2 Interpretation of gravity measurements
  • 3 Modern gravitational theory
  • 4 Acceleration of gravity on Earth’s surface →212A
  • 5 The gravitational constant, G:

The sub-sub-section 131B3 will have a green card, with the phrase ‘Modern gravitational theory’ on the top line, the classmark at top right, and the continuation of the full OoK caption text underneath the abbreviated 3-word caption given above, namely: ‘and its relation to other aspects of physical theory’ on the line(s) beneath. The two sub-sub-sub-sections are then listed, after a blank line, and with a space at the left:

  • a Field theories of gravity and their general properties and predictions
  • b Gravitational fields and the general theory of relativity: →D2.

Note the ‘see’ right-arrow ‘→’ in b redirecting to the nearby related sub-sub-section D2, which will require the creation of another green card as well as a yellow ‘parent’ card for the new green card. For now, we just create a blue card, with the classmark 131B3b, and the full caption, although with enough space left at the right end of the line directly underneath the 131B3b classmark for a red redirection →D2 to be placed directly underneath the classmark. This blue card is otherwise left blank and simply acts as a placeholder whose sole job is to redirect to 131D2, where the 131 is implied from the nesting level of the 131B3b card.

A yellow card is needed for 131D, which follows the pattern that has been described: classmark at top right, the caption shown on the orange 131 card placed on the top line (‘Gravitation:’), with the subtitle ‘the astronomical implications of relativity theory’ placed underneath that, a blank line, then the two sub-divisions:

  • 1 The special theory of relativity
  • 2 The general theory of relativity

In addition, an indicative dot is placed to the left of the D on the 131 card to show that section 131D is now in use, and another indicative dot is placed next to the 2 on the 131D card as well. With the creation of the 131D card, there is now a proper place for 131D2 to be created and filed without becoming an ‘orphan’ section (i.e., without a suitable ‘parent’).

However, before we do that, there is a new wrinkle to consider. As someone who has been a researcher in this field, I know that there are mathematical results that pertain to relativity theory, which may also be relevant to any research in this sub-discipline of physics. In the OoK, these are located at 023H6 Relativity theory, which is part of H Mathematical aspects of physical theories contained in 023 Applications of mathematics. There is in fact a ‘see also 131.D’ located at the OoK’s equivalent of 023H6 (i.e., 10/23.H.6) on p.491 but, strangely, no reciprocal ‘see also 10/23.H.6’ located at 131.D.

To distinguish the ‘see also’ cross-reference from the ‘see’ ‘→’ redirection, we use the ‘see also’ double-right arrow ‘⇒’ from the UDC. Therefore, to properly complete the yellow 131D card, we place the ‘see also’ cross-reference ⇒023H6 in red ink on the line directly underneath the 131D classmark. This means we will need to write the caption subtitle text in such a way as to leave enough space at the right side of the line to allow for the red cross-reference ⇒023H6 to be seen as clearly separate from the caption text.

Any 023H6 card which might be created at some later stage would automatically be prompted by the text on p.491 in the OoK to include the ⇒131D cross-reference. This is a ‘see also’ class cross-reference (as opposed to a ‘see’ class redirection), linking two related classes, and also as opposed to a zettel cross-reference which would link two related zettels.

The green card for 131D2 now follows the established pattern: classmark at top right, the caption from the 131D card repeated on the top line, and a blank line. Here, too, there are mathematical results from 023H7 Riemannian geometry which are relevant to research in general relativity. Hence, as above, we place the ‘see also’ cross-reference ⇒023H7 in red ink on the line directly underneath the 131D2 card’s classmark at top right.

Now we list the three sub-divisions of 131D2, as usual, leaving the customary blank line and space at the left side:

  • a Use of relativity to interpret gravitational phenomena
  • b Experimental confirmation of the theory
  • c Implications of general relativity

We now introduce a new sub-division that is not present in the OoK at this location. Underneath item c we now write, in blue ink to indicate that it is an addition to the OoK, the following:

  • d Mathematical extensions to GR

and place a dot to the left of the d, to indicate that it is now in use.

This then gives rise to a new blue card 131D2d with the title ‘Mathematical extensions to GR’ placed on the top line, and a subtitle ‘including Einstein’s unified field theory and other theories based upon GR’, all in blue ink.

This new section was added because this is where notes related to my doctoral thesis topic need to be filed, as well as for work on the semi-symmetric metric connection which I am reporting on in other posts. Both of these theories are based upon, but are mathematical extensions to, the general theory of relativity. It might end up being convenient to sub-divide this section even further according to the various theories that end up being filed here. These would therefore be purple cards, with classmark 131D2d1 for Einstein’s unified field theory (the first one I worked on), and 131D2d2 for the SSMC (the second). Any third theory would then get 131D2d3 as its classmark, and so on, all in blue ink. The 131D2d card would then need to be updated with listings of all of these, again in blue ink.

In my hardcopy volume of the Propædia, I then used a suitably-cut sliver of a Post-It Note, with the caption d. Mathematical Extensions to GR written on it, placed just underneath the end of the caption for c. Implications of general relativity, on p.52. (Any further sub-division into 1 and 2 would then also get a suitable sliver.) This location seemed more appropriate than a possible new section c at 131B3 since, although 131B3 deals with modern gravitational theory, this work is based specifically upon general relativity, which is redirected from 131B3b to 131D2. In general, filings and additions should be made at the sections to which ‘→’ re-directions from other parts of OoK are made. We would also place suitable Post-It Note slivers for the above cross-references, as well as one for ⇒131D2 at 023H7 for completeness. For ‘see also’ cross-references, we might also choose to file according to the ‘:’ or ‘::’ conventions from UDC (i.e., numerical order for ‘:’ and strict order for ‘::’ depending on whether the relationship is bi-directional or not).

The colour scheme for the structure cards obviously derives from the visible spectrum of colours from physics: red = long wavelength, which implies the coarsest resolution; purple = shortest wavelength, which implies the finest resolution, and the other colours in-between. Therefore, the index card colour indicates at a glance how far down we are in the classification hierarchy without having to check the number of digits in the classmark, as easy as that is. It also makes flicking through the cards a bit easier, since we know what colour card we are looking for, and how many of this colour to count inwards from the start of that section (i.e., how many ‘in-use’ dots are shown on the higher-level card).

To aid this process even more, small translucent coloured sticky flags 25mm wide can be affixed to the back side of the coloured index cards (so they don’t easily pull off when being pulled from the front, or obscure or smudge the writing). The index cards are ~150mm wide, so that 6 of these flags can fit across the width of a card—which is the same number of nesting levels that exist below the top-level Part. Therefore, by placing these flags in one of six positions on a coloured card, it is possible to subdivide the Part according to colour and therefore degree of nesting. First position at left-hand edge of a card is for the Division. Second position, 25mm along from the edge is for the Section. Third position, 50mm along is for the sub-section. Fourth position 75mm along is for the sub-sub-section. Fifth position 100mm along is for the sub-sub-sub-section, and sixth position 125mm along (or right edge) is for the sub-sub-sub-sub-section. The general layout therefore is: pink cards get pink flags placed in first position, orange cards get orange flags placed in second position, yellow cards get yellow flags placed in third position, and so on. Writing the classmark onto the flag also makes navigation just that bit easier.

In-section indices

It is also very useful, once a section begins to grow, to create an index within the section for any level of sectioning (from 2-digit to 7-digit classmarks). This is done on white lined index cards, with the classmark at top right, along with the separator character, and the number 0 given as the in-section ID #. Thus, they are filed directly behind the coloured structure card, and before any note-bearing zettels. If more cards are required than just this single index card, the second is numbered ‘0a’, the third is ‘0b’ and so on, following Luhmannian practice. If ‘interstitial’ index cards are ever needed, then numbering such as ‘0a1’ and similar can also be used, again following Luhmannian practice.

These in-section indices are related to but subtly distinct from the more general ZK-wide indices that provide wide-ranging indexing. The in-section index is for terms or themes specific to the particular section, which makes searching within the section more convenient when seeking to install a new zettel. But they also have indexing information which allows for more general ZK-wide index terms to link to relevant individual zettels. Typically, each new card ‘stream’ or ‘theme’ has a unique top-level number within the section.

For example, the white lined index card 001H1b|0 contains the themes which have emerged for ideas about the working materials for implementing a ZK, since this is the topic for the class 001H1b (see Part V for the captions for the 001 sections). These include notes about: index card box organising, such as the coloured-card structure described here in this post; the use of temporary ‘capture’ zettels of suitably-cut paper vs permanent cardstock cards; ‘hybrid’ ZK structure (part physical, part digital); and notes about implementing a physical system before considering a digital one. In particular, the quote at the top of this post from abramdemski is located at 001H1b|5a, and as such, is part of the 5th theme or card-stream that emerged in section 001H1b.

This way, one can skim the relevant in-section index card(s) to see where a new zettel might most sensibly be filed, either with relation to an earlier one in an existing theme/stream, or as the seed of a new potential theme/stream with a new top-level in-section ID number # within the relevant section. A ZK-wide index term in the main index under ‘Zettelkasten, working materials’ might link directly to the section 001H1b. But a more specific sub-term under this, say, ‘-,-, index card box organising’, might link to 001H1b|0.3 for the line-entry in the in-section index, or directly and equivalently to 001H1b|3 for the seed zettel of the multi-zettel theme, while the sub-term ‘-,-,-, coloured structure cards’ would link directly to 001H1b|3a, which may be or end up being a seed for its own series of sub-zettels. Linking to the in-section index cards allows for a more general orientating inspection of a related topic area than linking directly to specific zettels. This more general view might be more useful when seeking to generate connections across classes as well as between themes in different classes.

Going digital?

Hopefully this series of posts has been helpful to anyone looking to organise their research notes using physical index cards organised into knowledge disciplines based upon the timelines of Cosmic Evolution and Big History, using ‘increasing complexity’ as the primary sequencing index parameter.

However, if we take the comment of abramdemski above to heart, we might now consider implementing an electronic ZK using insights gained from the card-based implementation just described. There are, as one might imagine, some issues that come up with regard to the apparently straightforward idea of taking a physical ZK digital. This will be the subject of the (eventual) next post.

Until then, happy physical Zettelkasteneering.

Next Time: Part VII: Digital Zettelkasteneering


Adler, Mortimer J. 1994. “The Circle of Learning.” In Propædia: Outline of Knowledge & Guide to the Britannica, 15th edn, 30:5–8. The New Encyclopaedia Britannica. 32 vols. Chicago: Encyclopaedia Britannica Inc.

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