As cities grew, their internal economies became more complicated. People specialized in their labors, some producing grain, some pottery, some bricks, etc. A system of barter developed so that the wares and services of one individual could be exchanged for those of another. In response to the need for a medium of exchange, and for more sophisticated ways to store purchasing power and set standards of value, Currency came into use. The first satisfactory Currency was coinage made from electrum, a naturally occurring alloy of gold and silver.