The Toyota Production System and the scientific method that underpins it were not imposed on Toyota-they were not even chosen consciously. The system grew naturally out of the workings of the company over five decades. As a result, it has never been written down, and Toyota's workers often are not able to articulate it. That's why it's so hard for outsiders to grasp. In this article, we attempt to lay out how Toyota's system works. We try to make explicit what is implicit. We describe four principles - three rules of design, which show how Toyota sets up all its operations as experiments, and one rule of improvement, which describes how Toyota teaches the scientific method to workers at every level of the organization. It is these rules-and not the specific practices and tools that people observe during their plant visits-that in our opinion form the essence of Toyota's system. That is why we think of the rules as the DNA of the Toyota Production System. Let's take a closer look at those rules (for a summary, see the sidebar "The Four Rules").