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The Library of Congress uses both approaches, to an extent. The cataloguing system uses a hierarchical classification, based on one originally developed by Thomas Jefferson, on whose initial donation the Library of Congress is based. This is known as the Library of Congress Classification, and is used to specifically locate a given title or work within the stacks, that is, each item has one and only one location. There are also subject headings which are more tag-based, though also on a controlled vocabulary. A given work is given a (relatively small number) of subjects to which it's associated. These are not hierarchical, though of course the listing of subject headings itself follows a sequence. Unlike the classification, which assigns a single location to each work, the headings are a search aid to patrons searching for a set of related works within a subject heading, or facilitate branching of a search to possibly related subjects. Tagging systems, especially ad hoc tags supplied by untrained users, are popular but tend to produce numerous issues over time. Not that formal systems (as with the LoC systems mentioned here) are immune to same. One feature of the LoC systems is that they've evolved processes for managing change over time. Examples would be terminology or classifications which are now deprecated, or of regions and polities which have changed or no longer exist (e.g., the Austro-Hungarian empire, the USSR), or of changes in underlying classifications (e.g., of chemical elements or of biological classifications, both of which have evolved significantly over the life of the Library of Congress). The history of hierarchical information classifications is long and IMO fascinating, dating at least to Aristotle and his Categories, as well as numerous variants used in classifications of knowledge (such as Francis Bacon's) or encyclopedias, including Diderot's and Britannica. |