Antoninianus (Double Denarius)
The antoninianus is a Roman double denarius coin denomination (pl. antoniniani) struck from 215 to 293 A.D. (or 274 if you consider the later issues, sometimes called aurelianiani, a new denomination). On the obverse of the antoninianus the emperor is depicted wearing a radiate crown, caesars are bare-headed, and empresses are shown with a lunar crescent behind their shoulders. On some later antoniniani the emperor wears a helmet. The ancient name for the type is unknown. Our modern name for it, antoninianus, names it after Marcus Aurelius Antoninus (nicknamed Caracalla) who introduced it in A.D. 215. Although the antoninianus was valued at two denarii, the weight was considerably less than that of two denarii. During the reign of Gordian III the antoninianus replaced the denarius as the primary Roman denomination. The first antoniniani were first struck with an alloy containing about 49.5% silver, but the denomination was continually debased until by 274 the alloy contained only 2.5% silver. In 274, Aurelian reformed the radiate denomination, striking on a copper core with a 5% silver coating, and valuing the new coin at 5 denarii commune. Older antoniniani no longer circulated after this reform and the radiate coins struck after the reform of 274 until 293 are also called aurelianianus. The antoninianus (or aurelianianus) was used until reform of Diocletian in 293.