Requirements, architectures and risks

Modern software development processes, like the Rational Unified Process, prescribe an iterative approach to software development. A fundamental assumption of an iterative process is that system requirements don't have to be completely understood to commence development. At first glance the assumption that one can start developing a system without completely understanding its requirements seems paradoxical. However, upon closer inspection requirements can be divided into many categories, one of them being "architecturally-significant requirements". It is the understanding of these requirements, the associated development risks, and the system architecture that drive the early iterations of system development.