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The shipping container is one of the mainstays of international trade. The globalised modern economy depends on the rapid and efficient movement of goods that containerisation allows. In many ways it was the advent of the container that allowed this globalised economy to develop. Invented during World War two as an efficient method of moving equipment to the front lines, there are now at any one time up to 15 million containers being used to transport goods on land and sea or waiting to be filled at factories and ports. They are vital in the supply chain and have allowed the added efficiency of "just in time" inventory management, where companies no longer keep large warehouses of stock or parts, but rely on the ability to quickly order what they want from their suppliers. It is estimated that since the 1980s the ratio of inventory to GDP in American businesses has fallen from 25% to 15%. Altogether total business inventory in the US is estimated at $1.5 trillion, without "just in time" management methods this might be as much as $2.5 trillion. This means that companies rely more and more on the prompt delivery of parts from their suppliers to fulfill orders. This is particularly true of industries such as computer manufacture, which no longer make all the parts of the products that bear their names, but instead out source, often to suppliers half way around the world. American computer manufacturers are, for example, increasingly dependent on Asian microchip manufacturers in countries such as Taiwan and Thailand.