DDC

Because I’ve been sequestered at home for far too long, I've been rereading the classic history of the DDC, The Eighteen Editions of the Dewey Decimal Classification by John P. Comaromi. Comaromi was the editor in chief of Dewey from 1980 to 1991, and he describes the history of the classification in technical critiques of decisions made as well as describing the personalities involved. Reading his book is a great way to recognize all the multiple points at which the development of the DDC might have gone in different directions, if not for personality quirks or external pressures.

Since I'm at the part of the book where he's talking about one of my favorite things about the DDC, I'll take that as an opportunity to talk about add instructions. Add instructions were formerly referred to as "divide like" notes; the concept itself has been present since Edition 1 of the DDC back in 1876. Add instructions are absolutely the best thing about Dewey! But they're also an aspect that non-catalogers don't necessarily know about, since they can be challenging or impossible to spot without guidance provided by the schedules.

There are a few different types of add instructions that appear in Dewey. One of the most frequent is adding from one of the six Dewey tables. For example, at the number 737.49 Coins of specific countries, a note tells you to "Add to base number 737.49 notation […] from Table 2." So a work about Chinese coins would be classed at 737.4951 (that is, the base number 737.49 + the Table 2 number for China, 51).

A slightly different type of add instructions tells you to add one part of a number to another base number. For example, the number for astronomical works about moons is 523.98 Satellites and rings. At that number, you'll find the note "Add to base number 523.98 the numbers following 523.4 in 523.43-523.48." That's a little tricky to parse, but it's easier when you can look over at the 523.4 hierarchy and see that that's where the numbers for non-Earth planets in our solar system are found. Once you know that, for example, the number for Mars is 523.43, you can follow the instructions given above: Add to base number 523.98 the numbers following 523.4 in 523.43, that is, 3. So the class number for the moons of Mars is 523.98 + 3, that is, 523.983.

And my very favorite thing in Dewey is when you get the chance to add notation 001-999. That means that, at certain numbers, you can add the whole set of Dewey numbers (like, all of them, from 001 all the way to 999) to a base number! Let's see what that looks like in practice with the example 133.58 Application of astrology to specific topics. The add note there says "Add to base number 133.58 notation 001-999." That means that you can add any Dewey number to the base number to reflect what the astrology of that subject is. Do you have a book about astrology and crime? Just take the base number 133.58 and add the number for crime, 364.1, and smash 'em together, so the number for the application of astrology to crime is 133.583641. A book on the astrology of making pizza? 133.58 + 641.8248 = 133.586418248!