DDC

DDC is an acronym that can refer to several concepts, depending on the context. The two most widely encountered meanings are the Dewey Decimal Classification, a library classification system, and Direct Digital Control, a building automation technology.

The Dewey Decimal Classification (DDC) was devised by Melvil Dewey in the late 19th century to organize library materials by subject. It divides knowledge into ten main classes, each represented by a three-digit number, with further decimal subdivisions to specify topics. Libraries assign call numbers using the DDC to arrange items on shelves and to support cataloging and retrieval. The system uses additional Cutter numbers for author and title sequencing and is periodically updated to reflect new subjects and disciplines. DDC is used worldwide and is commonly integrated with library catalogs and management systems to maintain consistent organization across institutions.

Direct Digital Control refers to digitally based control of building systems, where sensors measure variables such as temperature or airflow and a central controller calculates control actions sent to actuators like valves, dampers, or fans. DDC enables closed-loop control, monitoring, scheduling, and energy management for HVAC, lighting, and other services. Controllers communicate over field networks and commonly use protocols such as BACnet, Modbus, or LonWorks, allowing integration with larger building management systems and smart-building applications.

Other uses for the acronym exist in specialized domains, but the Dewey Decimal Classification and Direct Digital Control are the most widely recognized meanings of DDC.