As cities grew, their internal economies became more complicated. People became specialists, some primarily producing grain, some pottery, some bricks, etc. A system of barter developed, so that one individual's wares and services could be exchanged for those of another. Gold and silver were widely used in such transactions, but the pieces needed to be weighed and tested for purity each time they changed hands. In 600 BC, the Lydians found a way around this problem. They began the practice of shaping electrum, an alloy of gold and silver, into pieces of predetermined weight and purity, stamped with an official symbol. This idea caught on, and within 50 years all the important trading centers of the world had adopted similar practices. The widespread use of currency created universal standards of value, allowing people from various societies to conduct business almost anywhere without being forced to bring along bulky goods for trade and barter.