The earliest Chinese proto-coins, as early as 770 - 476 B.C., were imitations of the cowrie shells used in ceremonial exchanges. The first metal coins, also introduced in this period, were not initially round; instead, they were knife shaped or spade shaped. Round metal coins with a round hole, and then later a square hole, in the center were first introduced around 350 B.C. The beginning of the Qin Dynasty (221 - 206 B.C.), the first dynasty to unify China, standardized coinage for the whole Empire. At first, coinage was limited to use around the capital city district but by the beginning of the Han Dynasty, coins were widely used for paying taxes, salaries, and fines. Ancient Chinese coins are markedly different from coins produced in the west. Chinese coins were cast in molds, unlike western coins which were typically struck (hammered) or, in later times, milled. Chinese coins were usually made from bronze, brass, or iron. Precious metals like gold and silver were uncommonly used. The alloys of the coin metals varied considerably. Most Chinese coins were produced with a square hole in the middle. At the mint coins were threaded on a square rod so that the rough edges could be filed smooth on a lathe, after which they were threaded on strings for ease of handling. Official coin production was sometimes spread over many mint locations throughout the country. Aside from officially produced coins, private coining was common during many stages of Chinese history. At times private coining was tolerated, sometimes it was illegal. Some coins were produced in very large numbers. During the Western Han, an average of 220 million coins a year were produced. Some other types were of limited circulation and are extremely rare today.