Faceted classification

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A faceted classification system allows the assignment of multiple classifications to an object, enabling the classifications to be ordered in multiple ways, rather than in a single, pre-determined, taxonomic order.

The most prominent use of faceted classification is in faceted navigation systems that enable a user to navigate information hierarchically, going from a category to its sub-categories, but choosing the order in which the categories are presented. This contrasts with traditional taxonomies in which the hierarchy of categories is fixed and unchanging. For example, a traditional restaurant guide might group restaurants first by location, then by type, price, rating, awards, ambiance, and amenities. In a faceted system, a user might decide first to divide the restaurants by price, and then by location and then by type, while another user could first sort the restaurants by type and then by awards. Thus, faceted navigation, like taxonomic navigation, guides users by showing them available categories (or facets), but does not require them to browse through a hierarchy that may not precisely suit their needs or way of thinking.

The Colon classification developed by S. R. Ranganathan and the Universal Decimal Classification are the two most prominent examples of faceted classification in the physical world, where for many years this approach to classification was thought of as something complicated, difficult to understand and exotic. The rules for generating class numbers are a key part of this complication, but they are necessary to make sure each item gets its proper spot on the bookshelf. In the online world, faceted classification becomes much more useful because it is not bound by this constraint. It makes information access useful, by providing multiple navigational paths to any one item of information.

In contrast to a folksonomy, the information in each of the facets can be organized into a hierarchy (for instance, the location facet could be divided by state, then cities, then neighborhoods). Also, folksonomies are emergent properties of social tagging systems in which individuals apply "tags" as they please, without control or coordination; faceted systems require someone to make a decision about which facets to record in the database and, often, which values will be permitted.

Faceted classification is also used in software engineering to support software reuse. It is also used outside of navigation systems to drive the dynamic display of the data in a database.