Many aspects of database systems have been improved by Graphical User Interfaces (GUIs). One area that has not received adequate attention in GUI research is the visual presentation of schemas, despite the increasingly important role that schemas play in database design and operation. Schema visualizations are valuable for viewing and manipulating both the schema and the information captured by it. In this paper, we describe a framework that formalizes the process of visualizing database schemas or any similar structured information. It is based on the concepts of adata model capturing schemas, avisual model capturing visualizations, and avisual metaphor that defines a mapping between the two models. This formal description of the mapping between a schema and its visualization permits straightforward declaration of visual metaphors, and provides criteria to evaluate metaphors as to their ability to correctly visualize a schema. Given a visual metaphor, the framework divides visual information into that which has meaning relative to the data model, that which has meaning to the user but not to the data model, and that which is only aesthetic. This separation permits better use of aesthetic information, resulting in richer visualizations. As a whole, we believe that the formalism provides the foundations on which better schema visualization tools can be built.