Software architecture is a recognized and indispensable part of system development. Software architecture is often defined in terms of components and connectors, or the "high-level conception of a system". In recent years, there has been an awareness that not only the design itself is important to capture, but also the knowledge that has led to this design. This so-called architectural knowledge concerns the set of design decisions and their rationale. Capturing architectural knowledge is difficult. Part of it is tacit and difficult to verbalize. Like developers, software architects are not inclined to document their solutions. Establishing ways to effectively manage and organize architectural knowledge is one of the key challenges of the field of software architecture. This architectural knowledge plays a role during development, when architects, developers, and other stakeholders must communicate about the system to be developed, possibly in a global setting. It also plays a role during the evolution of a system, when changes are constrained by decisions made earlier.