We call sites like this, where the link labels residing at the same level of depth in the directed acyclic graph (DAG) correspond to a facet (Wynar, 2000) of information categorization, faceted. Facets are ‘clearly defined, mutually exclusive, and collectively exhaustive aspects, properties, or characteristics of a class or specific subject. . . ., a term introduced into classification theory and given this new meaning by the librarian Ranganathan and first used in his Colon Classification in the early 1930s.

It was through the Colon Classification that Ranganathan introduced the concepts of facets and facet analysis. Facets were Ranganathan’s solution to the inherent problems with the Dewey Decimal and Library of Con- gress classification systems, namely, that index terms had to be conceived before an information object could be classified by the system. Rather than starting by designing a classification scheme and then fitting objects to it, Ranganathan’s idea was to first start with the objects and then dynamically cull the index terms and attributes from them to provide more flexibility and description accuracy in the classification. The core facets which Ranganathan believed could classify any object were: personality, matter, energy, space, and time (Taylor, 1999).

Faceted search techniques integrate navigational (e.g., Yahoo!) and direct search (e.g., Google) to help users determine which portions of a classification contain the information desired. In other words, they combine browsing and search, leading to a mixed-initiative mode of interaction (Perugini & Ramakrishnan, 2003). While faceted classifications enumerate all possible path permutations to each terminal object in the presentation of hyperlinks, faceted search does not enumerate any, but rather uses search to achieve multiple access paths to the same item. While complementary in this respect, both approaches can be used in concert (leading to what is called faceted browsing and search) and, if implemented properly, the two integrate seamlessly.