How to Find a Dewey Decimal Number
The Dewey decimal system, invented by Melvil Dewey in 1873, is used to classify books by discipline so that related books are usually grouped close to each other. This system is most commonly in place in public libraries, but some enthusiasts with extensive private libraries choose to employ the Dewey decimal system as well to keep their books organized. In order to gain access to every level of the Dewey system you much purchase the complete guide to it, but you can look up a Dewey decimal number to some degree of specificity for free online.
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Instructions
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Determine the main discipline of the book in question, if you're trying to find the Dewey number for a book, or the main discipline represented by the keywords for a book.
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Cross-reference the main discipline against the Ten Main Classes listed in the DDC 22 (see online at oclc.org/dewey/resources/summaries/#ten). For example, if you'd decided that the main discipline of a book is religion, its Dewey decimal number will be somewhere in the 200s, anywhere from 200 to 299.
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Classify the book or subject more specifically by cross-referencing against the "hundred divisions" in the Second Summary. These determine the second of the three digits you'll always see before the decimal point in the Dewey system. So if your book on religion deals with a religion other than Christianity, it would be classed in the 290s (anywhere from 290 to 299), as part of "Other Religions".
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Identify the last of the leading three numbers in the Dewey classification by further narrowing the topic according to the "Third Summary" given in the DDC 22. Just find the discipline or topic that most closely represents the subject you're looking up. For example, if the book you want to classify relates to Judaism, its Dewey decimal number would be 296.
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Tips & Warnings
You can also try looking up the subject or discipline you're curious about in the Appleton Public Library's Dewey lookup system at apl.org/libcats/dewey.html.
