 A particular distinguishing feature of diagrams is that they obey certain “nomic” or “intrinsic” constraints due to their use of plane surfaces as a medium of representation. The idea is that sentential languages are based on acoustic signals which are sequential in nature, and so must have a compensatingly complex syntax in order to express certain relationships—whereas diagrams, being two-dimensional, are able to display some relationships without the intervention of a complex syntax (Stenning &amp; Lemon  2001).  Diagrams exploit this possibility—the use of spatial relations to represent other relations. The question is; how well can spatial relations and objects represent other (possibly more abstract) objects and relations?