Although most ancient coins were struck on newly made flans, it was not unusual for a coin to be struck with an older coin used at the "blank" flan. Overstrikes are important because we can firmly establish the overtype is a later issue than the undertype. Overstrikes have been used to determine not just the sequence of issues but have also been used to more precisely date issues and, in some cases, to establish the order of rulers' reigns. For some ancient realms, numismatics provides the primary or only clues of who ruled and when they ruled. Sometimes overstrikes were done to recycle worn or obsolete coins. The most interesting overstrikes were done for political reasons. For example during the Jewish Bar Kochba revolt against Rome, the rebels struck their own silver zuz over Roman denarii, thus obliterating symbols of the hated Romans and replacing them with their own.