Construction of any kind of knowledge organization system starts from the fundamental building blocks, which are concepts. Once a representative set of concepts has been chosen and each one clearly defined, their hierarchical relationships can be established. Fundamental facets emerge from these hierarchies. In a post-coordinate scheme, such as a thesaurus, associative relationships between concepts in different facets can be defined. In a pre-coordinate scheme, such as a classification, concepts from two or more facets can also be combined, following rules of citation order, to express compound subjects. Citation order has to take account of the role which each concept plays in the compound as well as the facet to which it belongs, and these roles and facet names should be explicitly shown in schedules to distinguish such combinations from hierarchical subordination. The first-cited facet need not be discipline, or field of activity, but can, for example, be “things” or “phenomena”. Hierarchies which are not based on genus-species relationships need special consideration; whole-part relationships, for example, are best restricted to concepts which are individual instances rather than classes.