A comparatively homogeneous product that can typically be bought in bulk. It usually refers to a raw material – oil, cotton, cocoa, silver – but can also describe a manufactured product used to make other things, for example, microchips used in personal computers. Commodities are often traded on commodity exchanges. On average, the price of natural commodities has fallen steadily in real terms in defiance of some predictions that growing consumption of non-renewables such as copper would force prices up. At times the oil price has risen sharply in real terms, most notably during the 1970s, but this was due not to the exhaustion of limited supplies but to rationing by the OPEC cartel, or war, or fear of it, particularly in the oil-rich Middle East.