" Between each two work centers,6 and for each component separately, the accumulation of inventory is limited by setting a certain number of containers and the number of units per container. These containers, like every container in every industry, also contain the relevant paperwork. But, one page of the paperwork—usually a card (kanban in Japanese)—a page that specifies only the component code name and the number of units per container, is treated in an unconventional way. When the succeeding work center withdraws a container for further processing that card is not moved with the container, rather it is passed back to the preceding work center. This is the notification to that work center that a container was withdrawn, that the allotted inventory is not full. Only in that case is the preceding work center allowed to produce (one container of parts specified by the card). In essence the Kanban system directs each work center when and what to produce but, more importantly, it directs when not to produce. No card—no production. The Kanban system is the practical mechanism that guides the operation when not to produce (prevents overproduction). Ohno succeeded to expand Ford’s concepts by changing the base of the mechanism from space to inventory. "

― Eliyahu M. Goldratt , The Goal: A Process of Ongoing Improvement