How the Dewey Decimal Classification Works

Posted on Tuesday February 11, 2020

The Dewey Decimal Classification (DDC) is widely used in libraries to organize their collections. I think a lot of people have probably used the DDC to find a book in a library, and a lot of people generally know how it works: number ranges correspond to high-level topics, with more numbers in the middle to fill in more specific subjects. You might be familiar with the table of main classes:

I’ve always been interested in how the rest of the digits were decided on, so I decided to learn more! Surprisingly, it’s a bit challenging to find references on the DDC because it’s actually sort of a proprietary system. It’s managed and published by the Online Computer Library Center (OCLC), and they’re quite happy to sell you the DDC or access to WebDewey for many hundreds of dollars.

After some further digging, I came across an online class on the Dewey Decimal Classification from the Nebraska Library Commission. It’s three sessions of about an hour each. And now I know a lot more about how the DDC works!

The DDC was created in the 1870s by Melvil Dewey, who was a problematic person, and as a result the DDC has its share of issues. For these reasons and others, many libraries are moving away from the DDC to other systems such as the Library of Congress classification system or the BISAC subject codes used by many booksellers. Though it might be in decline, it’s widely-enough used that I still wanted to learn more about it.

The DDC organizes works into one of the ten main classes shown above. Each class has ten divisions (the second digit), and each division has ten sections (the third digit). There are further subdivisions that can be applied for more specific works. Overall, this forms a tree structure in which each subsequent digit traverses down the tree to a more specific topic. Works are classified into the node which is as specific as possible, so in general a shorter number or a number with fewer non-zero terminal digits will refer to a work that covers a broader range of topics.

In order to properly class works, there are two primary variables to consider: the subject/topic and the discipline. For example, you might class a work on dogs either in 599.77 (Natural sciences and mathematics > Animals > Mammals > Carnivores > Dog Family), or in 636.7 (Technology > Argiculture and related technologies > Animal husbandry > Dogs), depending on whether it was a book about the physiology of dogs or on keeping dogs as pets.

Of course, sometimes a work covers multiple topics or even multiple disciplines. The DDC has rules which dictate how these situations should be handled. (To continue the dog example, if you look in 599.77, there is a note which says “class interdisciplinary works on dogs in 636.7,” so if a work covered both the biology and raising of dogs, it should be classed in 636.7).

If you were to buy a hard copy of the DDC, you’d notice that there are a few different parts. The main part that people think of as the DDC is called the “schedules.” This is the big list of all the top-level numbers, arranged into chapters for each main class. There’s also an introduction, which has rules for deciding where works should be classed. For example the rule of fuller treatment says that if a work covers two or more topics, but covers one topic more fully than all the others, the work should be classed under that topic. There’s also the rule of two, which states that if a work covers two topics fairly equally, it should be classed under the lower number. For example, a work on with equal treatment of French bulldogs (636.72) and Welsh corgis (636.737) should be classed under the lower number, 636.72.

In addition to the introduction and the schedules, there’s also the manual which helps you resolve some specific situations (usually you’ll see a note in the schedules like “See manual 636.72-636.75” that points you to go there), the relative index, and the tables. The relative index is generally the starting point for classifying a work. You can look up a topic alphabetically, and you’ll be pointed to all the different possible classifications. And finally, the tables, which help classify works more specifically.

This introduces a topic called “number building.” The DDC doesn’t actually contain a specific entry for each possible topic, but relies on adding standard subdivisions to numbers listed in the schedules. Table 1 contains the standard subdivisions, which you can add as a suffix to pretty much any number you find in the schedules. The standard subdivisions include:

For example, an encyclopedia of programming languages could be classed under 005 (Computer programming, programs, data), .1 (programming), 3 (programming languages), —03 (Dictionaries, encyclopedias, concordances) to yield the number 005.1303. The book Cracking the Coding Interview, which is about the job of programming , could be classed under 005.1023, again using 005.1 (programming) and adding —023, the standard subdivision for “the subject as a profession, occupation, hobby.”

There are four tables in total; table 2 is used in conjunction with the 09 standard subdivision from table 1, e.g. a book about the architecture of Boston might be classed under 720.9744, with 720 being the architecture, 09 being the standard subdivision for geographic treatment, and 744 being the suffix from table 2 for Massachusetts. You might expect the number to be 720.09744, and it would be, except that in the schedules under 720, we are instructed to put the standard subdivisions in .1 through .9.

Table 3 contains subdivisions for literatures and literary forms and is only used with the main class 800 Literature. For example, a collection of American plays might be classed as 81 (American literature in English) + 3 (the subdivision from table 3 for Drama) to get 813 as the result.

Finally, table 4 contains subdivisions for languages, and is only used with the 400 Language main class. It’s used to break down specific attributes of language, such as —3 for dictionaries. So Webster’s dictionary would be classed as 420 (English and Old English) + 3 (dictionaries) = 423.

There’s a lot to the system, and while there is still a lot I don’t know, I now know a lot more about how it works than I did previously! If I got something wrong here, please email me about it! I’d love to learn more.