Finding Your Way Around a Library: Dewey Decimal Classification System

Engineers know the numbers 620 through 629 all too well from their time in the stacks.
Marcia Wendorf
Yale University Sterling Memorial LibraryHenry Trotter/Wikimedia Commons

If you studied engineering, then the numbers 620 through 629 may sound very familiar to you. They're the Dewey Decimal Classification system's designation for engineering subjects.

The Dewey Decimal Classifications are how we find resources within a library. The system was named after Melvil Dewey (1851-1931), who was a librarian at Amherst College after he received his master's degree from there.


In 1876, Dewey published his library classification scheme in a pamphlet entitled, A Classification and Subject Index for Cataloguing and Arranging the Books and Pamphlets of a Library. Today, only a single copy of the pamphlet survives. That same year, Dewey received a copyright for a subsequent edition of the index, and he, along with others, founded the American Library Association (ALA).

Dewey's classification system relied on positioning books in relation to other books on similar topics. Before Dewey, libraries in the U.S. assigned shelf positions to books based on the height of their spines, and their date of acquisition. Finding a book wasn't a problem for patrons since they weren't allowed in the stacks, which were only accessible to librarians.

By the early 20th century, patrons were beginning to browse the stacks, and by 1927, the Dewey system was being used by approximately 96% of public libraries, and by 89% of college libraries.

In 1930, the U.S. Library of Congress began printing Dewey Classification numbers on its catalog cards. In 1931, a French version of the Dewey system was published as the Universal Decimal Classification.

For generations, the library card catalog was a familiar sight to students and patrons. These huge cabinets contained hundreds of drawers that were filled with thousands of index cards, describing each of a library's holdings, whether they be books, sheet music, records, or works of art.

Library index card
Library index card, Source: ScottishTshirt/Wikimedia Commons

Today, the card catalog has been replaced by the online public access catalog (OPAC), which is an online database of the materials held by a library or a group of libraries.

How the Dewey system works

The Dewey Decimal Classification organizes library materials by discipline, such as Science, Technology and History. There are ten major classes, with each class being divided into ten divisions, with each division having ten sections.

Three whole numbers make up the classes and sub-classes, and decimals designate further divisions. The ten classes are:

  • Class 000 – Computer science, information & general works
  • Class 100 – Philosophy & psychology
  • Class 200 – Religion
  • Class 300 – Social sciences
  • Class 400 – Language
  • Class 500 – Science
  • Class 600 – Technology
  • Class 700 – Arts & recreation
  • Class 800 – Literature
  • Class 900 – History & geography

The Dewey system's structure is hierarchical, for example:

Natural sciences & mathematics
Analytic geometries
Metric differential geometries 
Finsler geometry 

In case you were wondering, a Finsler manifold is a differentiable manifold M where a (possibly asymmetric) Minkowski functional F(x,−) is provided on each tangent space TxM, that enables one to define the length of any smooth curve as a specific formula, but I bet you knew that...

A real strength of the Dewey Classification system is that it allows for new numbers to be constructed. For example, you could use 330, which is economics, add to it .9 for a geographic designation, then add to that .04 for Europe. The resulting number, 330.94, would be the designation for the European economy.

The overall Engineering and Applied Operations category is numbered 620, with subareas numbered:

  • 620 - Engineering & Applied operations
  • 621 - Applied physics
  • 622 - Mining & related operations
  • 623 - Military & nautical engineering
  • 624 - Civil engineering
  • 625 - Engineering of railroads, roads
  • 626 - Not assigned or no longer used
  • 627 - Hydraulic engineering
  • 628 - Sanitary engineering
  • 629 - Other branches of engineering

Today, many libraries employ a classification system other than Dewey for fiction, and books are shelved in alphabetic order according to the author's last name. The Library of Congress uses its own classification system called the Library of Congress Classification (LCC), and it is used by most research and academic libraries in the U.S.

One sexy room

In 1988, the Online Computer Library Center (OCLC) of Dublin, Ohio acquired the copyrights and trademark of the Dewey Decimal Classification system. In 2003, OCLC sued a hotel.

Library Hotel, Manhattan
Library Hotel, Manhattan Source: Library Hotel

Located near the New York Public Library's main branch lies the Library Hotel. Each of its ten guest floors is themed according to a major category within the Dewey Decimal Classification system. Each room is themed after a sub-category of Dewey. Each room contains 50 to 100 books in line with its theme.

The hotel's third floor is Dewey Classification 300 - Social Sciences, with rooms numbered:

  • 300.006 Law
  • 300.005 Money
  • 300.004 World Culture
  • 300.003 Economics
  • 300.002 Political Science

The fifth floor is Dewey Classification 500 - Math and Science, with rooms such as:

  • 500.006 Astronomy
  • 500.005 Dinosaurs
  • 500.004 Botany
  • 500.003 Zoology
  • 500.002 Geology
  • 500.001 Mathematics

The sixth floor, 600 - Technology includes rooms numbered

  • 600.005 Computers
  • 600.004 Medicine
  • 600.003 Management
  • 600.002 Manufacturing
  • 600.001 Advertising

OCLC sued the Library Hotel for trademark infringement, and the case was settled out of court, with the hotel's owner making a financial donation to OCLC that was used to promote children's reading.

So, if you're ever in New York City, and you want to stay in the Erotic Literature Room at the Library Hotel, it pays to know that it is room 800.001. You're welcome.

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