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All About Call Numbers: Dewey Decimal


A few weeks ago we featured a post about Library of Congress call numbers. But LOC call numbers only account for part of the Trinity Library collection. What about the rest of our books? How are they organized?

The remaining books have call numbers that start with numbers, rather than letters. They’re organized using the Dewey Decimal classification system.

Dewey Decimal call numbers look something like this:

A sample Dewey call number.

Start at the top and work your way down.

First, the number: The number tells you what a book is about. All Dewey call numbers are part of a hierarchy (categories within categories), with each digit representing increasingly specific information.

The first digit is called the main class, which is the broad category. In this case it is 600, or Technology.

The second digit is the division — a more narrow definition of the topic. In this case it is 630, or Agriculture.

The third digit is called the section. This particular number — 630 — doesn’t have a section (you can tell because it’s a zero). The book is just about agriculture, not some subcategory of agriculture. But if it were 631, the section would be Techniques, equipment, and materials. If it were 635 it would be Garden crops (Horticulture).

The numbers after the decimal point further organize the material by sub-topic, with each digit representing and increasingly specific category.

Check out our hierarchy tutorial for a visual look at Dewey numbers.

TIP: How do we put these numbers in order? Easy: like any other number. Dewey call numbers are simple to organize!

Next: the letter-and-number combination (G793). This is called a Cutter number, and corresponds to the author. The combination of letters and numbers helps us keep books organized by author’s last name: G7 might be Green; G3 might be Geary; G65 might be Graham. If you know the author’s last name, you know approximately where to look!

TIP: Treat the numbers behind the letter like a decimal: G3 before G65 (.3 before .65); G65 before G7 (.65 before .7), etc. This is where Dewey can get tricky.

TIP: The more authors you have in a given section, the longer the numbers get. Imagine you have 25 authors whose names begin with ‘S’. You have to fit them all into the S’s, which means you have to use more than just S1-S9. Aaron Smith (S5); Adam Smith (S52); Andrew Smith (S522); Betty Smith (S53); etc.

If this seems like a lot of confusing information — don’t worry about it. Even if you don’t know what a Dewey call number means, it should be easy to find on the shelf. Treat the number like a number, then let the Cutter number guide you to the author’s last name.

TIP: Believe it or not, a general topic may fall in more than one location, depending on how it’s talked about in a book. For example, how-to books on gardening would fall under 635 (Garden crops [Horticulture]), while the psychological  benefits of gardening might fall in Applied psychology (158) and the scientific discussion of certain plants might fall under Botany (580). Thus, if you want to know about garden plants, you can either (a) get books from all over the collection (for a broad understanding) or (b) approach with a specific angle in mind (e.g. psychology).

TIP: A basic understanding of call numbers should make it easier to find the materials you need. But even if you are a call number expert, we ask that you not put books back on the shelves once you’ve taken them down. Put them on one of the many tables and book carts found throughout the library, instead. This helps the librarians be sure that our books are being re-shelved with 100% accuracy. The more accurate we can be, the easier it will be to find things the next time you need them!

For detailed information on Dewey call numbers, read the Online Computer Library Center’s history and explanation of the Dewey Decimal system. Jump to page 9 for an extensive list of classes and hierarchies.

Practice your Dewey shelving skills with interactive games, practice tests, and puzzles. Then come to the library and put your new talents to work!

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