WTF is the Dewey Decimal Classification System?

What is the Dewey Decimal Classification System?

The Dewey Decimal Classification system (DDC) is a library classification scheme that organizes books by subject. It divides the world’s knowledge into 10 main classes, each with 10 subdivisions. These subdivisions, in turn, are divided into 10 sections. Then, we get wacky by adding decimals to divide the sections even further. Therefore, the DDC can go to infinity and beyond!

DDC follows a hierarchical structure, meaning you start with broad concepts and drill down to narrower topics. It uses the Arabic numeral system for notations, and of course, incorporates decimals.

Why is this important? Because the Dewey Decimal system is how we locate books in the library! It’s the basis for call numbers, which are alphanumeric notations assigned to each book. These notations give books a home on library shelves and tell us where to find them.

DDC also does wonders for browsing. Since the first part of a call number is a Dewey notation, books on similar subjects are shelved together for our convenience. To learn more about call numbers, see How to Find Books in the Library Using Call Numbers.

Who Uses the Dewey Decimal System?

According to the OCLC, the Dewey Decimal System is the most widely used classification scheme in the world.¹ Over 200,000 libraries in more than 140 countries are Dewey-ing it.

In general, public libraries and school libraries use the Dewey Decimal system. Most academic and research libraries, on the other hand, use the Library of Congress Classification. That’s why call numbers are wildly different depending on the type of library and classification system.

Melvil Dewey

The mastermind behind the DDC was Melvil Dewey (1851—1931), a professional librarian who just loved organization. He issued the first edition of the Dewey Decimal system in 1876, but it was years in the making. When he was just 22 years old, he saw the need for a new library classification system because the existing ones just plain sucked.

According to Dewey, “for months I dreamed night and day that there must be somewhere a satisfactory solution.” Obsessed much?

Anyway, he goes on to say, “After months of study, one Sunday during a long sermon…the solution flasht over me so that I jumpt in my seat and came very near shouting ‘Eureka’! It was to get absolute simplicity by using the simplest known symbols, the Arabic numerals as decimals, with the ordinary significance of nought, to number a classification of all human knowledge in print….”²

Let’s get one more zinger from Dewey about his system: “My heart is open to anything that’s either decimal or about libraries.”³ Dewey sure was devoted.

Limitations of the Dewey Decimal System

While Dewey did a huge service to libraries, we have to acknowledge that his system has limitations. DDC is supposed to encompass all human knowledge, but really, it’s Dewey’s Western view of knowledge. On top of that, Dewey believed in the superiority of the Anglo-Saxon race. 😬 Just taking a gander through his system shows these biases.

How to Dewey-It

Let’s take a look at the Dewey Decimal system in action. We’ll review the 10 main classes, the hundred divisions, and the thousand sections. Then, of course, we’ll check out some decimals.

10 Main Classes of the Dewey Decimal System

The 10 main classes cover the world of knowledge. The first digit in each three-digit number represents the main class. For example, 900 represents History & Geography.

The Hundred Divisions

Each of the 10 main classes is further divided into 100 divisions, for a total of 100. The second digit in the three-digit number represents the division. For example, 900 is History, and 910 is Geography and travel. Note: Not all of the numbers for the divisions have been used. In these cases, they’re labeled “unassigned.”

If you’d like a Totally Historic chart of the DDC down to the hundred divisions, download this PDF.

The Thousand Sections

The 100 subdivisions of the 10 main classes are each divided into 10 sections, totaling 1,000. The third digit in the three-digit number represents the section. See the examples below. Note: not all the numbers for these sections are used. In these cases, they’re labeled “unassigned.”

Since there are so many of these sections, I’ll give you a taste.

• 900 – History & Geography
  • 930 – History of the ancient world (to ca. 499)
    • 931 – China
    • 932 – Egypt
    • 933 – Palestine
    • 934 – India
    • 935 – Mesopotamia & Iranian Plateau
    • 936 – Europe north & west of Italy
    • 937 – Italy & adjacent territories
    • 938 – Greece
    • 939 – Other parts of the ancient world

Decimals in Action

Finally, let’s see how those famous decimals work.

• 100 – Philosophy & Psychology
  • 130 – Parapsychology and occultism
    • 133 – Specific topics in parapsychology and occultism
      • 133.4 – Demonology and witchcraft
        • 133.42 – Demonology
        • 133.43 – Magic and witchcraft
        • 133.44 – Spells, curses, charms

As you can see, the DDC is pretty easy to follow! And, despite its limitations, it’s a great way to organize books. Besides, if you mess with Dewey, your librarian may just freak out…

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Footnotes

1. OCLC, DDC 23 Summaries; OCLC stands for Online Computer Library Center, a global nonprofit library organization. It’s basically the big cheese in the library world. ↩︎
2. Dewey, “Decimal Classification Beginnings.” ↩︎
3. Manguel, The Library at Night, 59. ↩︎

Bibliography & Further Reading

*One of the following links is an Amazon affiliate link, meaning I earn a small commission if you make a purchase, at no cost to you.

Dewey, Melvil. “Decimal Classification Beginnings.” Library Journal 115, no. 11 (1990): 87-90.

Manuel, Alberto. The Library at Night. New Haven: Yale University Press, 2008.

OCLC. DDC 23 Summaries. Accessed January 24, 2026.