Rapide is an event-based, concurrent, object-oriented language specifically designed for prototyping system architectures. Two principle design goals are: (1) to provide constructs for defining executable prototypes of architectures and (2) to adopt an execution model in which the concurrency, synchronization, dataflow, and timing properties of a prototype are explicitly represented. This paper describes the partially ordered event set (poset) execution model and outlines with examples some of the event-based features for defining communication architectures and relationships between architectures. Various features of Rapide are illustrated by excerpts from a prototype of the X/Open distributed transaction processing reference architecture.