Customer demand extends back through the value chain and affects how a manufacturer deals with suppliers. Materials inventories also represent waste. Thus, managing supplier linkages is also vital to lean manufacturing. JIT Purchasing requires suppliers to deliver parts and material just in time to be used in production. Supply of parts must be linked to production, which is linked to demand. One effect of successful management of customer and suppliers linkages is to reduce all inventories to much lower levels. Since 1980, inventories in the United States have fallen from 26 to 15 percent of the gross domestic product