other would be "cash receipt", where cash is received. These two events are linked: a cash receipt occurs in exchange for a sale, and vice versa. The duality relationship can be more complex, e.g., in the manufacturing process, it would often involve more than two events (see Dunn et al. [2004] for examples). REA systems have usually been modeled as relational databases with entity-relationship diagrams, though this is not compulsory. The philosophy of REA draws on the idea of reusable Design Patterns, though REA patterns are used to describe databases rather than object-oriented programs, and are quite different from the 23 canonical patterns in the original designs pattern book by Gamma et al. Research in REA emphasizes patterns (e.g., Hruby et al. 2006). Here is an example of the basic REA pattern shown as an E-R diagram: The pattern is extended to encompass commitments (promises to engage in transactions, e.g., a sales order), policies, and other constructs. Dunn et al. (2004) provide a good overview at an undergraduate level (for accounting majors), while Hruby et al. (2006) is an advanced reference for computer scientists. Here is a diagram of an extended REA pattern (from Hruby et al. 2006) REA is a continuing influence on the electronic commerce standard ebXML, with W. McCarthy actively involved in the standards committee. The competing XBRL GL standard however is at odds with the REA concept, as it closely mimics double-entry book-keeping. REA is now recognised by The Open Group within the TOGAF standard (an industry standard enterprise framework), as one of the modelling tools which is useful for modelling business processes. Further reading Hruby, P., Kiehn, J., Scheller, C. V. (2006). Model-Driven Design Using Business Patterns. Springer. Dunn, C., Cherrington, J. O., Hollander, A. S. (2004) Enterprise Information Systems: A Pattern-Based Approach. McGraw-Hill/Irwin. Hollander, A. S., Denna, E., Cherrington, J. O. (1999) Accounting, Information Technology, and Business Solutions. McGraw-Hill/Irwin. Geerts, L. G., McCarthy, E. W. (2002, Vol.3) An Ontological Analysis of the Primitives of the Extended-REA Enterprise Information Architecture. The International Journal of Accounting Information Systems, pp. 1–16 Geerts, L. G., McCarthy, E. W. (2000) The Ontological Foundation of REA Enterprise Information Systems. Working paper, Michigan State University McCarthy, E. W. (July 1982) The REA Accounting Model: A Generalized Framework for Accounting Systems in a Shared Data Environment. The Accounting Review, pp. 554–78 A Reference Ontology for Accounting. Fallon, R. L., Polovina, S. (2013)