Cataloguing ‘as a whole’ (description and access by author and subject) was discussed only in Cutter's Rules of 1876 (Cutter, 1876), written for a dictionary catalogue, that is, organised in a single sequence of access by author and subject. Since then, such comprehensive rules have not been conceived and ‘cataloguing’ has referred only to descriptive cataloguing, while subject cataloguing, or subject indexing, has been treated separately (Foskett, 1996). Ranganathan argued persuasively for the greater usefulness of an integrated subject-based catalogue (Ranganathan, 1964). The formulation ‘descriptive cataloguing’ was coined in the 1940s in the Library of Congress context when subject cataloguing was separated from other procedures.

Over the years, subject indexing has followed its own path. For many years it was marked by confrontations, even polemical ones, between the promoters of alphabetically ordered verbal subject access – subject access using words and phrases, referred to traditionally as ‘subject headings’ – and those of notational access organised according to bibliographic classification systems. Specific sectoral and disciplinary situations and needs, related to scientific, technical and management resources and documentation in the academic field or public libraries, have required or favoured the creation and development of different systems of verbal and systematic indexing – systematic referring to systems using notations of classification systems. These systems pay due respect to cultural and especially linguistic differences, much more significant in the semantic field than in descriptive cataloguing. This has led to an extreme fragmentation that has deterred the attempt to reach agreements on indexing principles or rules. However, there are the notable exceptions of the IFLA Principles Underlying Subject Heading Languages (SHLs) (Lopes and Beall, 1999) and the ISO 25964: 2011–2013 standard for thesauri), as well as the hypothesis of unified codes. This was demonstrated also by the difficulties in including the subject theme in the FR family, in IFLA LRM, in ICP and in RDA in a more substantial form than as a simple citation.

At present, references to theoretical elaborations reach in-depth levels in specialised areas where professional and disciplinary skills other than those of libraries are involved, such as in the search of knowledge organisation systems. Some of these are functional for the representation and retrieval of resources for their information value.