A way of regulating the working relationships between two machines or operations to prevent overproduction and ensure balanced use of resources. 

In the illustration, both machines and the conveyor belt will start running only when three conditions are met: machine A is full, the conveyor belt contains the standard amount of work in progress (in this case, one piece), and machine B is empty. When those conditions are met, all three parts start moving. They then wait until the conditions are met again.