Toyota has formulated six rules for the application of kanban

1. Each process issues requests (kanban) to its suppliers as it consumes its supplies.
2. Each process produces according to the quantity and sequence of incoming requests.
3. No items are made or transported without a request.
4. The request associated with an item is always attached to it.
5. Processes must not send out defective items, to ensure that finished products will be defect-free.
6. Limiting the number of pending requests makes the process more sensitive and reveals inefficiencies.

via wikipedia
https://en.wikipedia.org/wiki/Kanban

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