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+---
+title: How the Dewey Decimal Classification Works
+---
+
+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:
+
+<table>
+  <tr>
+    <td>000</td>
+    <td>Computers and general information</td>
+  </tr>
+  <tr>
+    <td>100</td>
+    <td>Philosophy and Psychology</td>
+  </tr>
+  <tr>
+    <td>200</td>
+    <td>Religion</td>
+  </tr>
+  <tr>
+    <td>300</td>
+    <td>Social Sciences</td>
+  </tr>
+  <tr>
+    <td>400</td>
+    <td>Language</td>
+  </tr>
+  <tr>
+    <td>500</td>
+    <td>Math and Science</td>
+  </tr>
+  <tr>
+    <td>600</td>
+    <td>Technology</td>
+  </tr>
+  <tr>
+    <td>700</td>
+    <td>Art</td>
+  </tr>
+  <tr>
+    <td>800</td>
+    <td>Literature</td>
+  </tr>
+  <tr>
+    <td>900</td>
+    <td>History and Geography</td>
+  </tr>
+</table>
+
+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][class] 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][dewey], and as a result the DDC has [its share of issues][crit]. For
+these reasons and others, many libraries are moving away from the DDC to other
+systems such as the [Library of Congress classification system][loc] or the
+[BISAC subject codes][bisac] 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:
+
+<table>
+  <tr>
+    <td>&mdash;01</td>
+    <td>Philosophy and theory</td>
+  </tr>
+  <tr>
+    <td>&mdash;02</td>
+    <td>Miscellany</td>
+  </tr>
+  <tr>
+    <td>&mdash;03</td>
+    <td>Dictionaries, encyclopedias, concordances</td>
+  </tr>
+  <tr>
+    <td>&mdash;04</td>
+    <td>Special topics</td>
+  </tr>
+  <tr>
+    <td>&mdash;05</td>
+    <td>Serial publications</td>
+  </tr>
+  <tr>
+    <td>&mdash;06</td>
+    <td>Organizations and management</td>
+  </tr>
+  <tr>
+    <td>&mdash;07</td>
+    <td>Education, research, and related topics</td>
+  </tr>
+  <tr>
+    <td>&mdash;08</td>
+    <td>Groups of people</td>
+  </tr>
+  <tr>
+    <td>&mdash;09</td>
+    <td>History, geographic treatment, biography</td>
+  </tr>
+</table>
+
+For example, an encyclopedia of programming languages could be classed under
+<strong>005</strong>
+(Computer programming, programs, data), <strong>.1</strong> (programming),
+<strong>3</strong> (programming languages), <strong>&mdash;03</strong>
+(Dictionaries, encyclopedias, concordances) to yield the number 005.1303. The
+book <em>Cracking the Coding Interview</em>, which is about the job of
+programming , could be classed under 005.1023, again using 005.1 (programming)
+and adding &mdash;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 &mdash;3 for dictionaries. So Webster's dictionary would be
+classed as <strong>420</strong> (English and Old English) + <strong>3</strong>
+(dictionaries) = <strong>423</strong>.
+
+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.
+
+[class]: http://nlc.nebraska.gov/handouts/classmaterials/ddcsummer2014/dewey.html
+[dewey]: https://en.wikipedia.org/wiki/Melvil_Dewey#Controversies
+[crit]: https://en.wikipedia.org/wiki/Dewey_Decimal_Classification#Influence_and_criticism
+[bisac]: https://bisg.org/page/BISACSubjectCodes
+[loc]: https://en.wikipedia.org/wiki/Library_of_Congress_Classification