What Is the Dewey Decimal System?

The Dewey Decimal System is a way of arranging library books by subject: it divides knowledge into ten main classes numbered 000 to 900, and narrows each one with further digits and decimals. The number a book receives becomes the first line of its call number, so that books on the same subject stand side by side on the shelf.

Its formal name is the Dewey Decimal Classification, or DDC. Melvil Dewey published it in 1876, and it is maintained today by OCLC, the library cooperative. If you have ever found cookery books at 641 or history at 900, you have used it.

What the system is for

A library has two separate jobs: describing what it holds and deciding where each item stands. The catalogue does the first, and How Does a Library Catalogue Work? explains how. Dewey does the second.

Before schemes like it, many libraries used fixed locations. A book was assigned to a particular shelf in a particular case, and its mark recorded that spot. When the collection grew, the marks had to change or the arrangement broke down.

Dewey’s scheme gives books a relative location instead. The number describes the subject, not the shelf, so a new book on cooking slots in among the other cooking books wherever they happen to be. The whole sequence can move to a new floor or a new building and still make sense.

Who devised it, and who keeps it now

Melvil Dewey worked out the scheme while he was a student assistant in the library of Amherst College in Massachusetts, and published the first edition in 1876. In the same year he was among the founders of the American Library Association. His name stayed on the association’s highest award until 2019, when it was removed because of his documented record of racism, antisemitism and harassment of women.

The classification itself has been revised edition after edition since then. OCLC, the cooperative that also runs the shared catalogue described in WorldCat and Union Catalogues, now owns and maintains it. It is published mainly as an online edition, WebDewey, and an abridged version exists for smaller collections that do not need the finest subdivisions.

The ten main classes

Everything a library might hold falls into one of ten classes. These are their standard names, with a few examples of what you will find in each.

Each class divides into ten divisions, and each division into ten sections. That gives a hundred divisions and a thousand three-digit sections before a single decimal point appears.

How a Dewey number is built

A Dewey number moves from the general to the particular, one digit at a time. Cooking is the classic example:

• 600 Technology
• 640 Home and family management
• 641 Food and drink
• 641.5 Cooking

Science works the same way. 500 is science, 590 is animals, and 599 is mammals. Every added digit makes the subject narrower, so a longer number means a more specific book.

The scheme also has tables of standard endings that can be added to a number. One table covers places. A book on Italian cooking is typically classed at 641.5945: 641.59 gathers cooking from particular places, and 45 is the scheme’s code for Italy. The same code for Italy turns up in history, travel and art, which is what makes the system learnable once you have seen a few numbers.

How to read a call number on the shelf

The class number is only the first part of a call number. Below or after it comes an author mark, usually letters taken from the author’s surname. An illustrative call number for a cookery book by an author called Smith might be:

641.5 SMI

Larger libraries often use a more precise author code that mixes letters and digits, but the principle is the same: the class says what the book is about, the mark separates it from its neighbours.

Three habits make the shelves readable:

• Read the number as a decimal. The digits after the point are compared one at a time. So 641.5 comes before 641.56, which comes before 641.6.
• Then read the letters. Within the same number, books run alphabetically by author mark.
• Watch for prefixes. Many libraries add a short code above the number for a location or audience, such as reference or junior collections. Fiction is often shelved separately by author’s surname rather than by its Dewey number in the 800s.

Dewey or Library of Congress?

The other major scheme in English-language libraries is the Library of Congress Classification, developed at the Library of Congress for its own collection at the turn of the twentieth century. It uses letters for its main classes and numbers beneath them. Cookery, for instance, sits under TX.

Broadly, public and school libraries use Dewey and academic and research libraries use Library of Congress, especially in the United States. Dewey suits a broad, general collection with short, memorable numbers. Library of Congress was built for an enormous research collection and expands more comfortably into narrow specialist subjects, where Dewey numbers can grow long. Dewey is also used well beyond the English-speaking world and has been translated into many languages. The institutions behind each choice are described in Types of Libraries.

Where Dewey has been criticised

Every classification reflects the time and place it was made, and Dewey’s reflects an American, Protestant view of knowledge from the 1870s. The most cited example is the 200s. Most of that class, from 220 to 280, is given to the Bible and Christianity, while all other religions share the final ten numbers, 290 to 299.

Language and literature show a similar pattern. English, German, French, Italian, Spanish, Latin and Greek each have their own division, and most of the world’s other languages and literatures share the 490s and the 890s.

Later editions have revised treatments of religion, women, sexuality and Indigenous peoples where the original arrangement placed them awkwardly or marginally. Change is slow for a practical reason. Moving a subject to a new number means relabelling and reshelving every affected book in every library that adopts the change, so libraries often apply revisions selectively.

Why some libraries move away from it

Some public and school libraries have reorganised part or all of their collection into bookshop-style sections, an approach often called genrefication. Instead of numbered runs, readers find signed areas such as cooking, travel, true crime or graphic novels. Some libraries base these sections on BISAC, the subject codes used by the book trade.

The argument for it is browsing. Casual readers tend to look for a topic, not a number, and plain-language sections work the way bookshops have taught them to shop. It fits a wider change in how library floors are planned, described in Why Modern Libraries Look Different.

The argument against is precision. Broad sections hold fewer distinctions than a thousand Dewey sections, borderline books end up wherever someone decided, and the arrangement is local, so finding your way in one library teaches you nothing about the next. Many libraries settle on a hybrid: genre signs for fiction and children’s books, Dewey order for non-fiction, and the Dewey number kept in every catalogue record either way.

Sources and further reading

OCLC publishes an overview of the Dewey Decimal Classification and its current editions. The Library of Congress publishes the outline of its own classification. Wikipedia’s article on the Dewey Decimal Classification gives the history and the full list of divisions, with onward references.