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The Genealogist’s Dewey Decimal System

The Genealogist’s Dewey Decimal System

According to the website National Today, December 10th is Dewey Decimal System Day.[1] This offbeat “holiday” got my attention because it’s the perfect holiday for a genealogist. Knowledge of the Dewey Decimal System is an important tool to rely on when browsing the shelves of your favorite library.


What is the Dewey Decimal System? Dewey is a classification system that libraries use to organize their books. Nonfiction library books have a call number. Call numbers are the Dewy Decimal numbers plus three letters of the author’s last name.

Also known as the Dewey Decimal Classification, these numbers organize library materials according to ten knowledge groups:

00–099 General Works

100–199 Philosophy and Psychology

200–299 Religion

300–399 Social Sciences

400–499 Language

500–599 Natural Sciences and Mathematics

600–699 Technology

700–799 The Arts

800–899 Literature and Rhetoric

900–999 History, Biography, and Geography

Dewey Decimal numbers start with a two-digit group and then become more specific. The three numbers before the decimal point are the subject of the book. The numbers after the decimal point are subdivisions organized by subject and author.[2]



The ten groups listed above are subdivided further into specific subject groups.[3] Over 23,000 categories exist within those ten groups.[4] You can learn more about specific categories from the University of Illinois Library guide, Dewey Decimal System-A Guide to Call Numbers.

Do all libraries use the Dewey Decimal System? No. You may come across one other system, especially in academic libraries, which is the Library of Congress Classifications, another system of library classifications. A comparison of the two systems can be found on the University of Mississippi library website.

Dewey and Genealogy

Knowing the Dewey Decimal System benefits your library search. Obviously, you can search an online card catalog, and you should do that. But there is a benefit to in-person browsing and the Dewey Decimal System can help with that. Browsing helps you discover similar materials to the ones you are interested in. Whenever I research in a library, I have a list of materials I want to take a look at, but I also spend some time browsing the nearby shelves for materials I may have missed in the catalog or pique my research interest.

You can find online Dewey Decimal aids for the genealogist that can help you determine which Dewey Decimal numbers you would most be interested in based on your research. Look for numbers that include a location or a specific subject such as a religion, occupation, or event.

  • FamilySearch Research Wiki – Book and Film Numbers Used by the Family History Library: This helpful wiki page explains how the FHL uses the Dewey Decimal System.
  • The Midwest Genealogy Center’s brochure Dewey for Genealogists is a free PDF that provides Dewey Decimal numbers for states and Missouri counties.
  • The St Louis County Library website includes a clickable list of Dewey numbers for states and counties.
  • California’s San Mateo County Genealogical Society website has a list of Dewey numbers relevant to their library collection, but some of the subject numbers will be of interest to any genealogist researching a library collection. 

As mentioned before, not every library uses the Dewey Decimal System. One example is The Pender County Library (North Carolina) webpage Genealogy Classification System, which explains the system they use. It's important that before you go to a library, you verify what system they use and become more familiar with it. That will ensure your success in finding the materials you need.

Ready for a Library Trip?

Are you ready to go to the library? I'm anxious to spend some time in my favorite libraries. Part of preparing for a library trip is knowing exactly what they have for your research. Learning more about how a library organizes its collection can help you find items you might not normally locate with a simple online catalog search.


[1] “December Holidays,” National Today ( accessed 2 December 2020).

[2] “Finding Books,” Pratt Institute Libraries (,further%20by%20subject%20and%20author.: accessed 2 December 2020).

[3] “Dewey Decimal Classification,” Britannica ( accessed 2 December 2020).

[4] “Dewey Decimal system ion the 21st Century Library,” StateTech (,put%20his%20name%20on%20it.&text=Dewey%2C%20currently%20on%20its%2023rd,it's%20time%20for%20a%20change.: accessed 2 December 2020).


Gena Philibert-Ortega is an author, instructor, and researcher. She blogs at Gena's Genealogy and Food.Family.Ephemera. You can find her presentations on the Legacy Family Tree Webinars website.



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Some libraries are moving away from Dewey. It has some issues (it's well over 100 years old and somewhat archaic in how it classifies certain topics), so keep that in mind when visiting a library. For now, the one where I work is sticking with Dewey, but another one not far from us has done away with it in large sections of its collection.

In the UK and Europe, Dewey has been widely replaced by the Universal Decimal Classification (UDC). In practice, they are almost identical and even some librarians will tell you they are the same. They are not, but the differences are adaptions to accommodate languages other than English and to provide a detailed structure better suited to the way knowledge has evolved since the Dewey system was created.

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