Many library classification systems use a combination of a fixed, enumerative taxonomy of concepts with subordinate facets that further refine the topic.
[1] A faceted classification system uses a set of semantically cohesive categories that are combined as needed to create an expression of a concept.
[2] To the extent possible, facets represent "clearly defined, mutually exclusive, and collectively exhaustive aspects of a subject.
In the Colon Classification system, a book is assigned a set of values from each independent facet.
Ranganathan said that there are five fundamental categories that can be used to demonstrate the facets of a subject: personality, material, energy, space and time.
[12] The Universal Decimal Classification scheme was created at the end of the nineteenth century by Belgian bibliographers Paul Otlet and Henri la Fontaine.
The goal of their system was to create an index that would be able to record knowledge even if it is stored in non-conventional ways including materials in notebooks and ephemera.
[13] The UDC has an overall taxonomy of knowledge that is extended with a number of facets, such as language, form, place and time.
[5][14] After a study of the literature in the field, he created the classification with the following facets: Notation was solely alphabetic, with the sub-facets organized hierarchically using extended codes, such as "g Industrial equipment and processes", "ge Machines".