Many software systems do not have a documented system architecture. These are often large, complex systems that are difficult to understand and maintain. One approach to recovering the understanding of a system is to extract architectural documentation from the system implementation. To evaluate the effectiveness of this approach, we extracted architectural documentation from the Linux^TM kernel. The Linux kernel is a good candidate for a case study because it is a large (800 KLOC) system that is in widespread use and it is representative of many existing systems. Our study resulted in documentation that is useful for understanding the Linux system structure. Also, we learned several useful lessons about extracting a system's architecture.