An antoninianus is a silver coin used during the Roman Empire, equal to two denarii.
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The basic Roman coins that were used throughout the empire were the As, Dupondius, Denarius, Sestertius and Aureus. There were also vanity coins or coins of necessity such as the Antoninianus, created by Caracalla or the gold Solidus created by Constantine. Some coins were more common in the republic but seem to have gone out of "style" during the principate. An example of this is the Quinarius. Other coins were discontinued such as the very small quadrans which was discontinued by Marcus Aurelius.
The Romans had a number of coins which were valued on the basis of the as (plural asses), a bronze, and later copper, coin. Roman coinage changed over time, with some coins dropped and new ones introduced.The aureus (golden) was a gold coin valued at 25 silver denari (400 asses) an its size was similar to that of the denarius, but it was heavier as it was gold. Originally u8sed rarely, it was made more common by Caesar.the denarius (containing 10) was a small silver coin. Its value was originally 10 asses. It later became 16 asses. It was the most common coin in circulation.The quinarius was a small silver Roman coin valued at half a denarius (5 asses).The sestertius, or sesterce, (2 ½) was worth 2 ½ asses. During the Republic it was a small, silver coin issued only on rare occasions, later it was a large brass coin. ItThe dupondius (two-pounder) was a large brass coin valued at 2 assesThe semis (half) was a small bronze coin that was valued at ½ an as.The quadrans (a quarter) was a low-value bronze coin worth ¼ of an as.Caracalla introduced the antoninianus which was worth 2 denarii and slightly larger than this coin. It was initially silver, but was slowly debased to bronze. It replaced the denariusDiocletian introduced:The solidus was gold coin with an initial value of 1,000 denarii. It was issued in small quantities.The argenteus was a silver coin.The nummus was initially a large bronze coin. It later became low-value copper coins used in the Byzantine empire.
Over many years many emperors debased the coins (decreased the content of precious metal) to help finance their expenses. This amounted to devaluation and an increase in the amount of coins in circulation. This, combined with the issuing of personalised coins by several usurpers emperors during the crisis of the 3rd century, eventually led to the coins having virtually no value and to runaway inflation.
The silver Denarius (pl: Denarii) was the main coin in the Roman economy over much of its life. Other coins include the silver Antoninianus (worth 2 Denarii) and the gold Aureus (worth 25 Denarii)