Three major principles in the structure of knowledge organization systems are identified and discussed: hierarchical trees of types, sets of facets conforming to general categories, and series of levels of reality. Each principle can be present at various degrees in different systems. The three principles can interact between them in various ways, depending on priority choices in system construction. Examples are reviewed of different priority options adopted in classifications and thesauri, both special and general, and their effects are critically examined. It is found that levels of reality, although less often explicitly acknowledged than other principles, contribute an important tool for knowledge organization.