A visual signal system — typically a cord or button at each workstation — that allows any worker to stop the production line when they detect an abnormality. When the andon is triggered, a light illuminates on an overhead board, a team leader responds immediately, and the line stops until the problem is resolved. This is the operational expression of jidoka: building quality into the process rather than inspecting for defects afterward. In mass production, stopping the line is catastrophic — every minute of downtime costs thousands of dollars. In TPS, stopping the line is essential — letting a defect pass downstream is far more expensive than fixing it immediately. The andon system requires a culture where workers feel safe reporting problems, directly connecting to w-edwards-deming's argument about driving out fear. Eric Ries adapted the andon concept for startups as the "stop and fix" discipline.