From its very inception, the study of software architecture has recognized architectural decay as a regularly occurring phenomenon in long-lived systems. Architectural decay is caused by repeated changes to a system during its lifespan. Despite decay's prevalence, there is a relative dearth of empirical data regarding the nature of architectural changes that may lead to decay, and of developers' understanding of those changes. In this paper, we take a step toward addressing that scarcity by conducting an empirical study of changes found in software architectures spanning several hundred versions of 14 opensource systems. Our study reveals several new findings regarding the frequency of architectural changes in software systems, the common points of departure in a system's architecture during maintenance and evolution, the difference between system-level and component-level architectural change, and the suitability of a system's implementation-level structure as a proxy for its architecture.Index Terms-software architecture, architectural change, software evolution, open-source systems, architecture recovery.

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