Library patrons have innate expectations about how documents should be arranged. Useful classification schemes are those which conform to these expectations and are thereby psychologically comfortable. All schemes necessarily deviate from these expectations, but not to the same degree. The greater the divergence from this mental standard within a scheme, the greater the psychological discomfort the patron will experience and the less useful the patron will find it. Using as an example the discipline of anthropology, this article develops a measure of the deviation of library classifications from collocation in mental space.