Business in ancient Egypt was carried on by means of barter. Coined money did not appear until the Persians introduced the silver shekel of Darius. In earlier times a complicated scale of comparative values was in existence whereby the value of an animal, a manufactured object or a piece of agricultural produce could be expressed in terms of other articles. A cow, for example, would be exchanged for an agreed but socially acceptable number of bushels of grain or flax. Graduated weights of stone or metal, sometimes marked to show that they were officially certified as accurate, were used in conjunction with the balance, which was of the type pictured in the psychostasia.