A system which promotes the transfer of goods and services between people, either individuals or societies. The term 'trade' may be used to mean the same, but it often refers more specifically to the formalized economic relationships of modern societies. Three different forms of exchange can be found: reciprocity, redistribution, and market exchange. There are also different spatial patterns of traded items, which can reveal the mode of exchange. In 'down-the-line' exchange, a commodity is passed successively from one group to another even further away from its source. The pattern will show a distinct decline in the quantity of the item as distance from the source increases; the higher the value of the item, the further it will reach. In 'directional exchange', where a commodity is traded directly from its source to a distant point without any intermediate exchange, the pattern of decreasing quantities with increasing distance will be distorted with a local concentration. Primitive forms of exchange include barter, gift exchange, potlatch, and silent trade. Any interaction where something passes between people - goods, information, gifts, money, etc.