The aim of this paper is to examine how The Swedish Library Classification System, also known as the “SAB-system”, has classified popular music. The investigation is limited to the classification category of “Yx Jazz, Rock and Popular Music” within the SAB-system. The development of this category is studied. The main part of the paper deals with the history of popular music. A comparative analysis is done between the historical development of Yx and the history of popular music. The questions at issue are whether the Swedish Library Classification System has been able to follow the development of popular music; how the different styles of popular music have been inserted in the classification, and if it is possible to make improvements of the present classification. The result of the analysis shows that from 1984 the Swedish SAB-system has had a very detailed classification of popular music through category Yx, but that it demands a continuous revision of this classification to keep it up to date. The author also discusses the problems with classification of something that is not completely definable such as popular music. The author also makes some suggestions on how to improve the present classification of popular music in the Swedish Library Classification System.