Much recent research has pointed to the critical role of architecture in the development of a firm's products, services and technical capabilities. A common theme in these studies is the notion that specific characteristics of a product's design -for example, the degree of modularity it exhibits -can have a profound effect on among other things, its performance, the flexibility of the process used to produce it, the value captured by its producer, and the potential for value creation at the

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... stry level. Unfortunately, this stream of work has been limited by the lack of appropriate tools, metrics and terminology for characterizing key attributes of a product's architecture in a robust fashion. As a result, there is little empirical evidence that the constructs emerging in the literature have power in predicting the phenomena with which they are associated. This paper reports data from a research project which seeks to characterize the differences in design structure between complex software products. In particular, we adopt a technique based upon Design Structure Matrices (DSMs) to map the dependencies between different elements of a design then develop metrics that allow us to compare the structures of these different DSMs. We demonstrate the power of this approach in two ways: First, we compare the design structures of two complex software products -the Linux operating system and the Mozilla web browser -that were developed via contrasting modes of organization: specifically, open source versus proprietary development. We find significant differences in their designs, consistent with an interpretation that Linux possesses a more "modular" architecture. We then track the evolution of Mozilla, paying particular attention to a major "re-design" effort that took place several months after its release as an open source product. We show that this effort resulted in a design structure that was significantly more modular than its predecessor, and indeed, more modular than that of a comparable version of Linux. Our findings demonstrate that it is possible to characterize the structure of complex product designs and draw meaningful conclusions about the precise ways in which they differ. We provide a description of a set of tools that will facilitate this analysis for software products, which should prove fruitful for researchers and practitioners alike. Empirically, the data we provide, while exploratory, is consistent with a view that different modes of organization may tend to produce designs possessing different architectural characteristics. However, we also find that purposeful efforts to re-design a product's architecture can have a significant impact on the structure of a design, at least for products of comparable complexity to the ones we examine here.