Event processing rules may be prescribed in many different ways, including by finite state machines, graphical methods, ECA (event-condition-action) rules or reactive rules that are triggered by event patterns. In this paper, we present a model for defining event relationships for event processing rules. We propose a so-called correlation set allowing users to graphically model the event correlation aspects of a rule. We illustrate our approach with the event-based system SARI (Sense and Respond Infrastructure) which uses correlation sets as part of rule definitions for the discovery of event patterns. We have fully implemented the proposed approach and compare it with alternative correlation approaches.