This paper considers the boundary between the library and information science (LIS) specialist and the music specialist's approach to music classification, and shows how crossing this boundary enhances knowledge on both sides. Musical medium – in other words, the forces needed to rehearse or perform a musical work – is at the heart of music classification. Therefore, three examples of music medium phenomena will be considered from a combined LIS/music perspective. First, the "great" vocal/instrumental divide will be considered. The paper will explore how this fundamental musicological categorization is reflected in LIS classification schemes, and what happens when the categorization of a vocal/instrumental hybrid such as the choral symphony is attempted. The paper then considers the classification of musical instruments, comparing taxonomies within organology – the study of musical instruments – to LIS classification schemes for music. Part of this analysis will consider the categorization of instruments into "families", and how this differs between the music and LIS disciplines. This will be followed by an examination of the cross-fertilization between organology classification schemes and their bibliographic cousins, including an account of how the major organological taxonomy "Hornbostel and Sachs" crossed through the music boundary and into LIS territory. Through exploring these three specific examples of issues in classifying music medium, the general relationship between the LIS and music disciplines can be examined. The paper will reveal that this relationship takes on many different guises, and that crossing the LIS/music boundary – in both directions – is critical to understanding the fundamental elements of music classification.