Faceted classification is an analytic-synthetic classification scheme. It classifies objects using multiple taxonomies that express their different attributes or facets rather than classifying using a single taxonomy.
A faceted classification system allows the assignment of an object to multiple taxonomies (sets of attributes), enabling the classification to be ordered in multiple ways, rather than in a single, predetermined, taxonomic order. A facet comprises "clearly defined, mutually exclusive, and collectively exhaustive aspects, properties or characteristics of a class or specific subject". For example, a collection of books might be classified using an author facet, a subject facet, a date facet, etc. Arlene Taylor gives a good visual description of Faceted Classification. She states, “If one thinks of each of the faces of a cut and polished diamond as a facet for the whole diamond, one can picture a classification notation that has small notations standing for subparts of the whole topic strung together to create a complete classification notation”.
Faceted classification is used in faceted search systems that enable a user to navigate information along multiple paths corresponding to different orderings of the facets. This contrasts with traditional taxonomies in which the hierarchy of categories is fixed and unchanging. In other words, once information is categorized using multiple facets, it can also be retrieved using multiple facets. Thus, a user would not be restricted to one identifying search term in order to retrieve an item. He or she could use a single term or link together multiple terms which increases his or her chances of retrieving the exact information that is being sought 
Examples of faceted classification
The colon classification developed by S. R. Ranganathan is the most well known example of faceted classification applied to the physical world, specifically for the purpose of organizing library materials. Catalogers analyze the subject of the work and determine its various facets; then there is a synthesis that determines the call number. In the colon classification system, a book is assigned a set of values from each independent facet. This facet formula uses punctuation marks and symbols placed between the facets to connect them. Colon classification was named after its use of the colon. It differs from traditional library classification schemas like the Dewey Decimal System and Library of Congress classification system, in which each document has a unique assignment in a single, hierarchically organized classification system.
Ranganathan stated that hierarchical classification schemes like the Dewey Decimal Classification (DDC) or the Library of Congress Subject Headings are too limiting and finite to use for modern classification and that many items can pertain information to more than one subject. He organized his classification scheme into 42 classes. Each class can be categorized according to particular characteristics, that he called facets. 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. He called this the PMEST formula:
- Personality is the most specific or focal subject.
- Matter is the substance, properties or materials of the subject.
- Energy includes the processes, operations and activities.
- Space relates to the geographic location of the subject.
- Time refers to the dates or seasons of the subject.
Another example of a faceted classification scheme is the Universal Decimal Classification (UDC), the UDC is considered to be a complex multilingual classification that can be used in all fields of knowledge. 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. They also wanted their index to organize material systematically instead of alphabetically.
Vanda Broughton compiled a chart that showed how a collection of socks would be classified using faceted classification. In this chart, socks were categorized using five facets: color, pattern, material, function, and length. A user could potentially search under one or a combination of these terms to find the desired product, unlike more traditional systems where only predetermined keywords would provide the desired results
Another example of faceted structure was presented in a study done by Mystic Seaport. The author presents the concept of air-to-ground missile as a part of a hypothetical knowledge scheme. After researching different categorization methods, it was shown that only in a faceted scheme would additional terms exist allowing the user to also search for ground-to-air missile without having to begin a new search under a different term. In a facet scheme, these terms would exist: air, ground, missile and this would allow users to easily fit together each type of classification: air-to-ground missile [and] ground-to-air missile.
The HyperSet system  formalized faceted classification using set theory, and implemented a prototype retrieval system, many of whose features are implemented in the Tinderbox personal content manager.
Advantages and disadvantages
Faceted classification systems can incorporate the wide variety of materials and catalog them. Both the LCC and the DDC were developed to organize print materials. As electronic library collections grow, a faceted classification system has the ability to incorporate all types of material and the ability to present it easily to the user.
The DDC is simpler than using a faceted classification system such as Ranganathan’s colon classification. DDC is more intuitive and makes much more sense to patrons browsing through a library’s stacks than another system would, and any class number developed through colon classification would be very long and complicated. A book can also only have one place on a shelf, whereas a multi-faceted sorting scheme would identify several shelves on which a book could be placed, necessitating the use of pointers or some other mechanism.
Comparison between faceted and single hierarchical classification
Hierarchical classification refers to the classification of objects using one single hierarchical taxonomy. Faceted classification may actually employ hierarchy in one or more of its facets, but allows for the use of more than one taxonomy to classify objects.
- Faceted classification systems allow the assignment of multiple classifications to an object, and enable those classifications to be applied by searchers in multiple ways, rather than in a single, predetermined order. Multiple facets may be used as a first step in a search process. For example, one may start from language or subject.
- Hierarchical classification systems are developed classes that are subdivided from the most general subjects to the most specific.
- Faceted classification systems allow for the combination of facets to filter the set of objects rapidly. In addition, the facets can be used to address multiple classification criteria.
- A faceted system focuses on the important, essential or persistent characteristics of content objects, helping it to be useful for categorization of fine-grained rapidly changing repositories.
- In faceted classification systems one does not have to know the name of the category into which an object is placed a priori. A controlled vocabulary is presented with the number of documents matching each vocabulary term.
- New facets may be created at any time without disruption of a single hierarchy or reorganizing other facets.
- Faceted classification systems make few assumptions about the scope and organization of the domain. It is difficult to break a faceted classification schema.
Faceted classification systems are also distinct from folksonomies or other tagging systems that do not break out the tags into independent facets.
- Classification Research Group
- Controlled vocabulary
- Information architecture
- Tag (metadata)
- Universal Decimal Classification
- Taylor, A. G. (1992). Introduction to Cataloging and Classification. 8th ed. Englewood, Colorado: Libraries Unlimited.
- Taylor, Arlene G. (2004). The organization of information. Westport, CT: Libraries Unlimited.
- Star, S.L. (1998, Fall). Grounded classification: grounded theory and faceted classification. [Electronic version]. Library Trends. 47.2, 218.
- Garfield, E. (1984, February). A tribute to S.R. Ranganathan, the father of Indian library science. Essays of an Information Scientist, 7, 37-44.
- Chan, L.M. (1994). Cataloging and classification. New York: McGraw-Hill, Inc.
- Colon Classification (6th Edition) by Dr. S.R. Ranganathan, published by Ess Publications, Delhi, India.
- Ranganathan, S. R (1987). Colon classification, 7th ed. revised and edited by M.A. Gopinath. Bangalore: Sarada Ranganathan Endowment for Library Science, 1987
- About universal decimal classification and the udc consortium. (2006). Retrieved November 30, 2013, from http://www.udcc.org/about.htm
- Batty, D. (2003). Universal decimal classification. Encyclopedia of Library and Information Science.
- Ranganathan and Facet Analysis. (2007). Mystic seaport. Retrieved November 29, 2013, from http://library.mysticseaport.org/msitia/facets.html
- H.V.D. Parunak,"Don't Link Me In: Set Based Hypermedia for Taxonomic Reasoning." Proc. Hypertext 1991, San Antonio, TX, 1991, pp. 233-242
- Glassel, A. (1998). End users corner: Was Ranganathan a yahoo!? Retrieved November 30, 2013,from http://scout.wisc.edu/Projects/PastProjects/toolkit/enduser/archive/1998/euc-9803.html
- Endeca at the NCSU Libraries, from https://www.lib.ncsu.edu/endeca/
- Sirovich, Jaimie (2011). Categories, Facets—and Browsable Facets?, from http://www.uxmatters.com/mt/archives/2011/08/categories-facetsand-browsable-facets.php
- Reitz, Joan M. (2004). Dictionary for library and information science. Westport, CT: Libraries Unlimited
- Godert, Winfried. F. (1991). Facet classification in online retrieval. International Classification, 18, 98-109
- Adkisson, Hiedi P. (2005). Use of faceted classification. Retrieved December 1, 2013, from http://www.webdesignpractices.com/navigation/facets.html
- How to Make a Faceted Classification and Put It On the Web - an article by William Denton
- How to Reuse a Faceted Classification and Put It On the Semantic Web
- Carl Linnaeus Invented The Index Card. ScienceDaily. Retrieved September 9, 2010, from http://www.sciencedaily.com /releases/2009/06/090616080137.htm - a mention of Linnaeus's use of faceted classification in the original index card system
- OpenMeta tagging system for the Mac - a (currently) flat system for making facets. (Requesting links for hierarchical tagging systems)