Universal Decimal Classification

People use a special code for books.

These codes help us find things. They group all kinds of facts together. This helps us learn new things.

Two men made this system long ago. They wanted to organize all knowledge. They used numbers to label items. These numbers can describe many things. They can describe books or even films.

One code can show many details. It can show a place or a time. This makes it easy to search. Many libraries around the world use it. It helps people find what they need.

How do we organize all the things people know?

Two men named Paul Otlet and Henri La Fontaine created a way to do this. They made the Universal Decimal Classification, or UDC. This system uses numbers to label all branches of knowledge. It is not just for books. It can also label films, maps, and even museum objects.

The UDC works by using special codes called notation. These codes use numbers to show a subject. The longer the code, the more detail it has. You can add more numbers to make a code more specific.

One great part of UDC is how it joins ideas. It uses common auxiliary tables. These are lists of general ideas like place or time. You can add these to a main subject code. For example, you can combine a history code with a place code. This tells you the history of a specific country. Today, about 150,000 libraries in 130 countries use this system to help people find information.

How do we organize everything people know?

The Universal Decimal Classification, or UDC, is a system for this hard job. It is a way to arrange all branches of human knowledge. This system makes sure that different fields of study are linked together. It works for many things, not just books. You can use UDC codes for films, maps, and even museum objects. It helps people find exactly what they need in huge collections.

The way it works is through special codes called notation. These codes use numbers to show a subject and its place in a group. The UDC uses a decimal system with Arabic numerals. A very long code means the subject is very specific. If you remove the last number, you get a broader subject. You can also combine different codes using special punctuation marks. For example, a colon can show two different ideas joined together. This allows the system to describe very complex topics.

Two men named Paul Otlet and Henri La Fontaine started this work. They lived at the end of the 19th century. In 1895, they created the Universal Bibliographic Repertory, or RBU. They wanted a complete index for all published information. An American named Herbert Haviland Field suggested using a card catalogue. Otlet also studied the Dewey Decimal Classification to help his plan. This led to many new ideas that made the UDC unique.

The UDC is used in about 150,000 libraries across 130 different countries. It is also used in national bibliographies in around 30 countries. Some large databases use it to organize millions of records. For instance, the VINITI RAS database has 28 million records. The 2011 version of the Master Reference File has over 70,000 classes. In the past, printed editions had about 220,000 subdivisions. This system is still updated today to include new knowledge.

You can think of the UDC like a giant, connected map of ideas. The main subjects are split into nine large classes from 0 to 9. Class 4 is left empty to leave room for future growth. There are also common auxiliary tables with over 15,000 entries. These tables hold general ideas like a place or a language. You can attach these to any main subject code. This makes it easy to talk about the history of a specific place.

The Universal Decimal Classification, or UDC, is a sophisticated system for organizing human knowledge. It acts as a bibliographic and library classification tool that arranges all branches of study into a coherent, inter-linked structure. Unlike many other systems that began as national tools, the UDC was designed from the start to be an international scheme. It is an analytico-synthetic and faceted system, which means it can break subjects down into parts and then combine them. This allows for very powerful content indexing and information retrieval in massive collections.

Today, the system is managed by the UDC Consortium, a non-profit international association of publishers based in The Hague, Netherlands.

To understand how the UDC works, you must look at its notation. A notation is a specific code used to represent a subject and its position within a hierarchy. The UDC uses Arabic numerals arranged in a decimal system. Every number is treated as a decimal fraction, even though the initial decimal point is omitted. This decimal structure is very useful because it is infinitely extensible. When new subjects are discovered, new subdivisions can be added without disturbing the existing numbers. For ease of reading, these notations are usually punctuated after every third digit.

The system features two very important expressive qualities: it is hierarchically expressive and syntactically expressive. Being hierarchically expressive means that the length of the code determines its specificity. A longer notation describes a very specific class, while removing the final digit produces a broader class. Being syntactically expressive means that UDC codes can be combined using precise punctuation marks. For example, a colon indicates that two distinct notational elements are being joined. You might use this to show a relationship between two different subjects, such as Law and Politics. Other symbols, like parentheses or quotes, can be used to add details about a place, a time period, or the physical form of a document.

Knowledge in the UDC is organized into two main types of tables. The first group consists of nine main classes, numbered 0 through 9. These classes cover everything from Science and Knowledge (0) to Geography and History (9). Interestingly, class 4 is left vacant. This was a planned decision made in the 1960s to leave space for the rapid growth of natural sciences and technology. The second group is the common auxiliary tables. These contain over 15,000 concepts that represent general characteristics, such as place, language, or time. Because these are "aspect-free," they can be added to almost any subject from the main tables. This allows a librarian to describe the history of the United Kingdom or the geography of the United Kingdom using the same basic concepts.

The history of the UDC began at the end of the 19th century. Belgian bibliographers Paul Otlet and Henri La Fontaine developed the system. In 1895, they created the Universal Bibliographic Repertory, known as the RBU. Their goal was to create a complete classified index of all published information. A young American zoologist named Herbert Haviland Field suggested they use a card catalogue for this project. Otlet also studied the Dewey Decimal Classification and received permission to translate it into French. However, the project grew far beyond a simple translation. The creators introduced radical innovations, such as the ability to synthesize compound numbers for interrelated subjects that could not be foreseen in advance.

The impact of the UDC is visible in its massive global scale. It is currently used in approximately 150,000 libraries across 130 different countries. It also serves as the main classification system for information exchange in several nations and is used in national bibliographies in about 30 countries. The system manages enormous amounts of data. For example, the VINITI RAS database contains 28 million records, and the Slovenian National Union Catalogue holds 3.5 million. The 2011 version of the UDC Master Reference File included over 70,000 classes. In the past, full printed editions were even larger, containing around 220,000 subdivisions.

Beyond just organizing books, the UDC is a vital tool for modern information science. Its design makes it highly suitable for machine readability. This has allowed it to work with early mechanical sorting devices and modern online library catalogs. It is also used to index a wide variety of media, including films, sound recordings, maps, and even museum objects. Because it is a disciplinary classification, it covers the entire universe of knowledge. It allows the same concept to appear in different fields of study by placing it under different perspectives. This makes the UDC a foundational system for connecting how we understand the world through different lenses of science, art, and history.