[dredmorbius]

Keywords also have their limitations.

The US Library of Congress is an interesting case study to my mind. The original classification scheme came from Thomas Jefferson's private library (he donated the collection to the US Government after the original Library of Congress was burned in 1812. The classification has been made more detailed (though so far as I know the original 20 alphabetic top-level classes remain as Jefferson established them), and there's been considerable re-adjustment, as knowledge, mores, and the world around us have changed. The classification has its warts, but it's also very much a living process, something I feel is greatly underappreciated.

At the same time, the Library also has its equivalent of keywords, the Library of Congress Subject Headings. Whilst a book or work will have one and only one Classification assigned to it (the Classification serving essentially as an index and retrieval key), there may be multiple Subject Headings given (though typically only a few, say 3--6 for a given work). These are used to cross-reference works within the subject index.

The Subject Headings themselves date to 1898, and there is in fact an article on the ... er ... subject, "The LCSH Century: A Brief History of the Library of Congress Subject Headings, and Introduction to the Centennial Essays" (2009), I'm just learning as I write this comment:

<https://www.tandfonline.com/doi/abs/10.1300/J104v29n01_01>