It is important that all those who use representations of a system during the development process can clearly understand the representations that are used. Research has shown that structure plays an important role in whether a diagrammatic representation may be readily understood. In this paper we present the results of a study where two different approaches were taken to the evaluation of two notations which form part of the UML diagram toolkit: sequence diagrams and collaboration diagrams. First, a theoretical investigation was carried out using the cognitive dimensions framework. Second, an empirical study was carried out to investigate user understanding of such diagrams. The results of the two studies did not concur, with the theoretical approach supporting the original hypothesis that structure is an important factor in diagram comprehension, but the study providing no evidence to support this
