All complex systems can be described in terms of their architecture, that is, as a nested hierarchy of subsystems. Despite a wealth of research highlighting the importance of understanding system architecture, however, there is little empirical evidence on the actual architectural patterns observed across large numbers of real world systems. In this paper, the authors developed robust and reliable methods to detect the core components in a complex system, to establish whether these systems possess a core-periphery structure, and to measure important elements of these structures. Overall, the findings represent a first step in establishing some stylized facts about the structure of real-world systems.