The Universal Decimal Classification ([[UDC]]) system is a bibliographic and library classification system that was first developed by the Belgian bibliographers [[Paul Otlet]] and [[Henri La Fontaine]] in the late 19th century. It is an extension and modification of the Dewey Decimal Classification (DDC) system. The UDC system organizes knowledge into a systematic hierarchy of concepts, enabling efficient cataloging and retrieval of information in libraries and other information repositories. It covers a wide range of subject areas, including science, technology, social sciences, humanities, arts, and more. Unlike the DDC system, which uses a decimal notation based on decimal numbers for classification, the UDC system utilizes a mixed notation that combines Arabic numerals with symbols. This allows for greater flexibility in expanding and modifying the classification scheme as new knowledge emerges. The UDC system consists of ten main classes denoted by numerical digits from 0 to 9. Each main class is further divided into subclasses using additional numerical digits or symbols. The specific arrangement of subclasses within each main class is based on logical relationships between subjects. One notable feature of the UDC system is its extensive use of auxiliary tables, which provide additional information for classifying specific topics. These tables include facets such as geographical regions, time periods, languages, forms of material (e.g., books, journals), and more. The UDC system has been widely adopted by libraries around the world due to its flexibility, adaptability, and applicability to various subject areas. It has undergone numerous revisions and updates to reflect changes in knowledge over time. In addition to its use in libraries, the UDC system has also been employed in other fields such as information retrieval systems, bibliographic databases, document management systems, and digital libraries. # Conclusion Overall, the Universal Decimal Classification system provides a comprehensive framework for organizing knowledge resources in a systematic manner while facilitating efficient access to information across different domains. # References [[@VisualAnalyticsResearchAndyVan2019|Reflections on a Half-Century of Hypertext]] see [15:10](https://www.youtube.com/live/g0yx-F1FGnc?si=CSlHWFGxSwp-qkxd&t=910)