The Dewey Decimal Classification (DDC) system stands as one of the most widely used library classification schemes globally, providing a systematic method for organizing knowledge. Developed by Melvil Dewey in 1876, the DDC has undergone numerous revisions and expansions to accommodate the ever-growing and evolving landscape of human knowledge. DDC 21, published in 1996, represented a significant milestone in this ongoing evolution, offering a refined and expanded framework for libraries worldwide. It built upon the foundational principles of its predecessors while introducing important updates and enhancements that improved its flexibility and comprehensiveness.

At its core, DDC 21 aimed to provide a logical, hierarchical, and easily navigable structure for intellectual works, ensuring that materials on similar subjects are grouped together and that the relationship between broader and narrower topics is clearly articulated. Its design facilitated consistent application across diverse library environments, from small public libraries to large academic institutions. Understanding the characteristics of DDC 21’s structure is crucial for appreciating its functionality, its impact on library operations, and its enduring legacy in the field of library and information science.

Characteristics of DDC 21 Structures

The structure of DDC 21 is characterized by several fundamental principles and components that work in concert to achieve its organizational objectives. These include its hierarchical arrangement, its pure decimal notation, the comprehensive scope of its main classes, the detailed nature of its auxiliary tables, and the sophisticated mechanisms for number building and specificity.

Hierarchical Arrangement

The most prominent characteristic of DDC 21, inherited from its initial design, is its hierarchical structure based on the decimal principle. All knowledge is divided into ten main classes, each represented by a three-digit number. Each of these main classes is then further subdivided into ten divisions, and each division into ten sections. This successive division from general to specific allows for an intricate and logical progression of subjects.

For example, the 500s represent “Natural sciences and mathematics.” Within this broad category, 510 is “Mathematics,” 520 is “Astronomy,” and so on. Taking 510 “Mathematics” as an example, it further subdivides into 511 “General principles of mathematics,” 512 “Algebra,” 513 “Arithmetic,” and so forth. The number of digits increases as the subject becomes more specific. This hierarchical progression means that works on a broader topic will have a shorter, more general class number, while more specific works will have longer numbers, placing them directly below their broader categories on the shelves and in catalogs. The “0” suffix is consistently used to denote general works within any given class, division, or section (e.g., 500 for general science, 510 for general mathematics, 511 for general principles of mathematics). This systematic arrangement ensures that users can easily browse collections and locate related materials, moving from general surveys to highly specialized treatises within a logical continuum.

Pure Decimal Notation

DDC 21 employs a pure decimal notation system using only Arabic numerals. This notation is the backbone of its structure, providing an unambiguous and infinitely expandable system for representing subjects. The use of a decimal point after the third digit (e.g., 301.36) allows for infinite expansion of numbers to represent increasingly specific topics. While the initial three digits represent the main class, division, and section, subsequent digits after the decimal point allow for further specification without predetermined limits, thereby accommodating new knowledge and emerging interdisciplinary fields.

The notation inherently reflects the hierarchy: longer numbers indicate greater specificity and are subsumed under shorter, more general numbers. For instance, 300 is “Social sciences,” 301 is “Sociology & anthropology,” and 301.3 is “Communities.” A work classified as 301.36 “Urban communities” is understood to be a specific type of community, nested within the broader concept of “communities,” which in turn falls under “sociology,” and ultimately “social sciences.” This mnemonic aspect of the notation, where patterns in numbers often relate to patterns in subject matter, assists both classifiers and users in navigating the system. The pure numeric system also makes DDC universally recognizable and interpretable across different linguistic boundaries, a significant advantage for international libraries.

Ten Main Classes (First Summary)

DDC 21 organizes all human knowledge into ten main classes, often referred to as the First Summary. These broad categories ensure that every conceivable subject finds a place within the system. The main classes are:

  • 000 Computer science, information & general works: Covers subjects like computer science, library and information science, journalism, and general encyclopedias.
  • 100 Philosophy & psychology: Encompasses metaphysics, epistemology, ethics, logic, and various branches of psychology.
  • 200 Religion: Deals with all forms of religion, including mythology, Christianity, other religions, and religious studies.
  • 300 Social sciences: Covers sociology, political science, economics, law, public administration, social problems and services, education, and commerce.
  • 400 Language: Includes linguistics, specific languages, and dictionaries.
  • 500 Natural sciences & mathematics: Broadly covers mathematics, astronomy, physics, chemistry, earth sciences, biology, botany, and zoology. Notably, DDC 21 introduced significant revisions to the 560-590 Life Sciences schedule, making it more logical and consistent with modern biological taxonomy.
  • 600 Technology (Applied sciences): Encompasses medicine, engineering, agriculture, home and family management, and manufacturing.
  • 700 Arts & recreation: Includes fine arts, decorative arts, music, performing arts, sports, and games.
  • 800 Literature: Covers literature of specific languages, specific literary forms (poetry, drama, fiction, essays), rhetoric, and literary criticism.
  • 900 History & geography: Deals with history, geography, biography, and general history of various regions.

These ten main classes serve as the primary entry points into the DDC system, providing a high-level overview of the classification’s scope and structure.

Divisions and Sections (Second and Third Summaries)

Beyond the ten main classes, DDC 21’s structure extends to more granular levels through its Second and Third Summaries. Each main class is divided into ten divisions (Second Summary), and each division is further divided into ten sections (Third Summary). For instance, within 300 Social sciences:

  • 330 Economics (Division within 300)
    • 331 Labor economics (Section within 330)
    • 332 Financial economics (Section within 330)
    • 333 Land economics (Section within 330)

This systematic subdivision creates a highly detailed and finely granulated classification system. The explicit enumeration of these divisions and sections in the DDC schedules allows classifiers to quickly locate the appropriate base number for a subject before applying further refinements. These summaries are crucial for maintaining the logical flow and consistency of the classification, ensuring that related subjects are kept together at progressively narrower levels of specificity.

Auxiliary Tables (Standard Subdivisions)

One of the most powerful and distinctive structural features of DDC 21, fundamental to its flexibility and capacity for specificity, is its set of auxiliary tables. These tables provide standard numbers that can be added to any base class number from the main schedules to express recurring characteristics, such as form, place, time, or language. DDC 21 featured six primary tables, each serving a distinct purpose:

  1. Table 1: Standard Subdivisions (-0): This is the most frequently used table. Numbers from Table 1 are added using a single 0 (or double 0, as in -001, -003, etc., where the double 0 acts as a separator for concepts that apply broadly across disciplines, like “systems” or “research”). They denote recurring characteristics of the subject, such as:

    • -01 Philosophy & theory
    • -02 Miscellany (e.g., directories, handbooks)
    • -03 Dictionaries, encyclopedias
    • -04 Special topics (a “general special” device)
    • -05 Serial publications
    • -06 Organizations & management
    • -07 Study & teaching; research
    • -08 History & description with respect to kinds of persons
    • -09 Historical, geographic, Persons treatment

    For example, a dictionary of chemistry would be 540 (Chemistry) + T1-03 (Dictionaries) = 540.3.

  2. Table 2: Geographic Areas, Historical Periods, Persons (-1 through -9): This table provides numbers for specific places, geographical features, and historical periods. These numbers are added to base numbers primarily for geographical treatment or historical context. They are often introduced by an instruction to “add notation from Table 2” with a specific base number.

    • -1 Areas, regions, places comprehensive (e.g., Earth, continents)
    • -3 Ancient world
    • -4 Europe
    • -5 Asia
    • -7 North America
    • -9 Oceania, polar regions

    For instance, the history of education in France would combine 370 (Education) with T2-44 (France) to form 370.944.

  3. Table 3: Subdivisions for Individual Literatures; for Specific Literary Forms (-1 through -8): This table is used primarily with the 800 (Literature) class. It provides numbers for literary forms (e.g., poetry, drama, fiction) and periods within specific literatures.

    • -1 Poetry
    • -2 Drama
    • -3 Fiction
    • -4 Essays
    • -5 Speeches
    • -6 Letters

    For example, American poetry would be 811 (American literature + T3-1 for poetry).

  4. Table 4: Subdivisions of Individual Languages and Language Families (-1 through -9): This table is used with the 400 (Language) class to specify aspects of particular languages, such as grammar, etymology, or dictionaries of that language.

    • -1 Writing systems & phonology
    • -2 Etymology
    • -3 Dictionaries
    • -5 Grammar
    • -7 Standard usage

    A dictionary of the French language would be 443 (French language + T4-3 for dictionaries).

  5. Table 5: Racial, Ethnic, National Groups (-1 through -9): This table provides numbers for specific racial, ethnic, or national groups, which can be added to a class number where the subject relates to a particular group.

    • -1 People (general)
    • -3 Specific ethnic and national groups (e.g., Germans, French)
    • -9 Specific racial, ethnic, and national groups (e.g., Black people, Chinese)

    For instance, a work on the education of African Americans might combine 371.829 (Education of specific groups) with T5-96073 (African Americans in the U.S.).

  6. Table 6: Languages (-1 through -9): This table provides numbers for specific languages (e.g., English, French, German) and is used when the work is in a specific language or about the language itself. This is distinct from Table 4, which details aspects of a language.

    • -1 Indo-European languages
    • -2 English
    • -3 German
    • -4 French
    • -5 Italian

    For example, a book of physics written in French might be 530 (Physics) + T6-4 (French language) = 530.04. However, it’s more common for DDC to focus on the subject and use a language code or MARC field for the language of the text. Table 6 is typically used when the language is the subject, such as a bibliography of works in French on a topic.

The tables significantly enhance DDC’s capacity for specificity and allow classifiers to construct highly precise numbers without having to list every possible combination within the main schedules. The instructions within the schedules guide classifiers on how and when to apply these table numbers, often using “add notes” or “add from” instructions.

Relative Index

DDC 21 includes a comprehensive Relative Index, an alphabetical listing of subjects, concepts, and relationships, accompanied by their corresponding class numbers. Unlike a simple keyword index, the Relative Index is “relative” because it not only points to the main classification number for a subject but also lists other numbers where aspects of that subject might be treated from different perspectives.

For example, “Water” might appear with numbers in Chemistry (546.22), Hydrology (551.48), Environmental Engineering (628.1), Biology (577.14), and so on. This feature acknowledges the interdisciplinary nature of knowledge and helps classifiers identify the most appropriate context for a given work, ensuring that all facets of a subject are properly located within the classification scheme. The Relative Index is an indispensable tool for initial subject identification and for resolving classification ambiguities, guiding classifiers to the correct schedules.

Manual

The DDC 21 Manual provides detailed explanations, interpretations, and practical advice for applying the classification schedules and tables. It contains a wealth of supplementary information, including:

  • Instructions for classifying difficult or ambiguous topics.
  • Clarifications of scope notes and “class here” notes.
  • Guidance on the preferred order of precedence for number building when multiple options are available.
  • Examples of complex number construction.

The Manual is critical for ensuring consistent classification decisions across different libraries and for providing expert insights into the nuances of the DDC system. It effectively serves as a practical guide for applying the theoretical structure of DDC to real-world library materials.

Number Building and Synthesis

A key structural characteristic of DDC 21, crucial for its flexibility and specificity, is its emphasis on “number building” or “synthesis.” Instead of providing a pre-defined number for every conceivable specific subject, DDC 21 provides instructions within its schedules that allow classifiers to construct highly specific numbers by combining elements from the main schedules with numbers from the auxiliary tables or even other parts of the main schedules.

This is achieved through:

  • Add Notes: Instructions to “add notation” from a specific table, or from another part of the DDC schedule, to a base number. For instance, an instruction might say “Add to 362.196, notation 001-999 from 616.1-616.9.” This allows for the classification of specific diseases within social welfare.
  • Order of Precedence: For complex subjects that could be classified in multiple ways, the DDC provides rules for determining the primary focus or “facet” of the work, guiding the classifier to the most appropriate base number and the sequence of added elements.
  • Faceting: While DDC is not a pure faceted classification system, it incorporates elements of facet analysis by allowing classifiers to combine different aspects (facets) of a subject – such as form, place, time, or specific subject characteristics – to create a comprehensive class number. This synthetic capability makes DDC a powerful tool for organizing highly specific and interdisciplinary materials.

Phoenix Schedules and Revisions in DDC 21

DDC 21 notably incorporated several “phoenix schedules,” which are complete rewrites of certain sections of the classification. These rewrites are undertaken when a particular field of knowledge has evolved so significantly that its existing DDC representation becomes unwieldy or illogical. For DDC 21, a major phoenix schedule was implemented for 570-590 Life Sciences (Biology, Zoology, Botany). This revision modernized the biological sciences schedules, aligning them with contemporary scientific understanding and taxonomic classifications, making the structure more logical and easier to use for new scientific literature. Another significant revision was the expansion of Public Administration and Military Science (350-354), reflecting growing needs in these areas. These substantial revisions demonstrate DDC’s commitment to adaptability and its continuous effort to reflect current knowledge organization paradigms. Relocations and reductions of existing topics were also frequent, ensuring greater logical consistency and eliminating redundancies.

Transition to Electronic Format and User Access

While DDC 21 was primarily known as a multi-volume print set, its publication marked a significant point in the transition to electronic access. DDC 21 was the first print edition to be released concurrently with “Dewey for Windows,” an electronic version. Although not a structural change to the classification itself, this digital interface profoundly impacted how users interacted with and navigated the DDC’s complex structure. Dewey for Windows offered enhanced search capabilities, hypertext linking between schedules, tables, and the index, and the ability to easily browse hierarchies. This innovation, accompanying DDC 21, made the intricate relationships within the DDC structure more transparent and accessible, laying the groundwork for subsequent electronic versions like WebDewey and greatly enhancing the practical application of the DDC’s internal logic.

Conclusion

The DDC 21, published in 1996, firmly established itself as a highly refined and adaptable knowledge organization system, maintaining the core principles of its predecessors while embracing enhancements crucial for its continued relevance. Its structure is fundamentally characterized by a logical and intuitive hierarchical arrangement, facilitated by a pure decimal notation that allows for infinite expansion and precise specificity. This allows libraries to organize their collections from the broadest disciplinary categories down to highly granular subjects, ensuring coherence and ease of discovery.

The indispensability of DDC 21’s structure lies in its comprehensive Main Classes, which provide a complete intellectual map of human knowledge, coupled with the sophisticated and versatile Auxiliary Tables. These tables empower classifiers to construct nuanced and precise classification numbers for complex subjects, reflecting multiple facets such as form, geography, time, or specific characteristics. The supporting tools, including the Relative Index and the Manual, further enhance the system’s usability and consistency, guiding classifiers through the intricacies of number building and subject placement.

In essence, DDC 21’s structural characteristics collectively delivered a robust and flexible framework for information organization. Its emphasis on logical progression, synthetic number building, and continuous adaptation through features like phoenix schedules ensured its continued utility in a rapidly expanding information environment. DDC 21 solidified the system’s status as a cornerstone of library classification, demonstrating its capacity to evolve while retaining the core principles that have made it a global standard for organizing and accessing knowledge.