The paper addresses issues in establishing a user-friendly systematic collection arrangement following a merger of two high school and college library collections classified according to UDC. In the way it was used, this scheme presented some weaknesses with respect to collection usage. Due to the disciplinary nature of UDC, subjects and phenomena are dispersed in the scheme according to the disciplines in which they are the subject of study. At the same time students in a school library often seek interdisciplinary subjects and need access to clusters of documents which according to UDC may be classed in several different knowledge areas. The author illustrates how this problem was resolved by re-arranging the collection according to phenomena. This was achieved by interpreting UDC numbers as if they represented specific phenomena. Thus, by superimposing some local indexing rules onto a disciplinary knowledge organization system it was possible to collocate interdisciplinary subjects under a single class number. Furthermore, by reversing subject numbers and form auxiliaries (atlases, dictionaries, textbooks etc.) which is an option envisaged in the design of UDC, documents were collocated in the way they are most frequently used by students. The author suggests that, in practice, one often needs to overcome the constraints of disciplinary classification and he discusses the approach used in his school library collection.