Architectural decisions (such as selecting patterns, technologies, or decomposing systems) and their rationale are a significant aspect of architectural knowledge. In this paper, we present a tool that helps architects capture tacit knowledge about architectural decisions. Furthermore, the tool helps architects and other stakeholders analyze decisions. Finally, the tool supports group decision making. The tool is based on theoretical and conceptual foundations created and evaluated in previous work. We developed the tool as a research tool in an academic environment, and we used the tool with industrial practitioners. The tool is web-based and available as an open source project. In this paper we highlight the underlying research contributions of the tool and show how research approaches and findings have been transferred into a working tool.