To create an understanding of enterprises and the ways they do business, a starting point could be to identify the main actors and the values transferred between them. Business models are created in order to make clear who the business actors are in a business case and to make their relations explicit. The relations are formulated in terms of values exchanged between the actors. The purpose of the work reported in this paper is to create a better understanding of business models by identifying basic notions used in such models. It does so by constructing a common ontology based on three established business model ontologies: e3 -value, REA, and BMO. By means of a careful analysis of these ontologies a conceptual schema is created that defines the common concepts. An example is worked out that explains how the common ontology should be understood.