Ancient Chinese coinage includes some of the earliest known coins of all time. These coins, used as early as the Spring and Autumn period (770–476 BCE), took the form of imitations of the infamous cowrie shells that were used in ceremonial exchanges. The same period also saw the introduction of the first metal coins; however, they were not initially round, instead being either knife shaped or spade shaped. Round metal coins with a round, and then later square hole in the center were first introduced around 350 BCE. The beginning of the Qin dynasty (221–206 BCE), the first dynasty to unify China, saw the introduction of a standardised coinage for the whole Empire. Subsequent dynasties produced variations on these round coins throughout the imperial period. At first, the distribution of the coinage was limited to use around the capital city district, but by the beginning of the Han dynasty, coins were widely used for such things as paying taxes, salaries and fines.