Today searching for information using words and phrases from natural language has become the norm. Everybody knows how to use a search engine, and most people will start a search by entering the name of whatever it is they wish to find out about. Although directory style methods of organizing information on websites are also common, many searchers find it simplest and easiest just to put some appropriate words into a search box. The existence of so much information in electronic format reinforces this approach, since it is easy to process huge quantities of text very rapidly, mechanically matching strings of characters to locate and retrieve the sought terms.

This increasing preference for language-based retrieval is mirrored in managed information resources, databases, indexes and bibliographies, and library catalogues. While the classification scheme was the dominant means of organization and retrieval in libraries for two-thirds of the twentieth century, with the arrival of the first automated systems in the 1950s and 1960s, there was an accompanying move towards language as the basis of indexing, and the emergence of tools such as keyword lists and thesauri. These have continued to be the norm for electronic collections or for collections where physical access and browsing are not important. In the library context, where automated retrieval by means of an online catalogue is combined with a substantial physical collection of documents, the subject heading list has usually been considered a more appropriate system.

This is particularly noticeable in the UK, where the use of subject headings in library catalogues has become widespread over the last 20 years. A number of factors have doubtless contributed to this situation: greater awareness of, and access to, subject headings through the medium of web-based catalogues; the advantages of bibliographic information sharing made possible by this phenomenon; the consequent need for standardization, particularly of the bibliographic tools used; and the ease of retrieval using words, as noted above. Why the subject heading list, rather than the thesaurus, should be preferred is not altogether clear, but it is now very well entrenched in most academic libraries of the developed world, the UK being one of the few countries not to have its own national system of subject headings.