The Qin introduced common coinage, meaning a form of ´fiat´ currency backed by the central government. It was called a ´ban liang´, and they could be issue by local nobles as well as by the emperor. A system of coinage is important for the unification of a massive empire because it facilitates trade and the recollection of revenue for the central government. As such, it becomes easier to raise a common army, which is essential for unification of different warring states.