Study of the organization of knowledge and of theories, systems, and practices of library classification provides a foundation for considering the design of an effective scheme. Use of existing standard systems has disadvantages and requires a library to either adapt itself to a standard or adapt that standard to the library. Prompt reclassification to an effective system will produce ultimate economy in service. Helpful order within a system is secured by application of certain principles. The scheme should be plastic to accommodate differing requirements and include alternative locations and citation orders of components. Construction and application of a relative classification present persistent problems.