Sestertius

The sestertius was a Roman coin and, above all, the main Roman unit for counting money. Under the Republic it was a small silver coin worth two and a half asses; from the reign of Augustus it was a large brass coin worth a quarter of a denarius. Roman prices, fortunes and state accounts were usually expressed in sestertii, written with the abbreviation HS.

The name comes from semis tertius, "the third a half", a Latin way of saying two and a half, which was the coin's original value in asses. Its large size and detailed reverse designs have made the imperial sestertius one of the most studied and collected of all Roman coins.

The first sestertius was a tiny silver coin struck alongside the denarius from around 211 BCE. As a quarter of the denarius, which was then worth ten asses, it was marked IIS, meaning two and a half. It was struck only occasionally and in small numbers, but the name lived on as the standard unit of account. When the denarius was retariffed at sixteen asses in the 140s BCE, the sestertius as a unit of reckoning was redefined at four asses.

Around 23 BCE Augustus reorganised Rome's base-metal coinage. The sestertius became a large coin of orichalcum, a golden-coloured brass of copper and zinc, typically weighing about 25 to 27 grams and more than 30 millimetres across. It was the largest coin in regular circulation and served as an everyday medium of exchange for larger market purchases. The two-as dupondius was also made of brass while the as was copper, so colour as well as size helped users tell the coins apart. Imperial bronze coins usually carried the letters SC, for senatus consulto, "by decree of the Senate", although the emperor controlled the coinage in practice.

Under Augustus the main denominations related as follows:

The broad surface of the sestertius allowed engravers to show complex scenes. Emperors used it to publicise military victories, harbours, temples and public works, along with personifications such as Concord and Abundance. Nero's sestertii showing the harbour at Ostia and the sestertius of Titus depicting the Colosseum are well-known examples, and for some buildings such coins are among the best surviving images.

Because its official value was well above its intrinsic value as metal, the sestertius worked partly as token money, closer in this respect to modern fiat currency than the precious-metal aureus or denarius. The gap between face value and production cost was a source of seigniorage for the state.

The sestertius kept a fairly stable weight through the first and second centuries CE, although its brass gradually contained less zinc and more lead. In the third century, as the silver coinage suffered severe currency debasement, the sestertius became lighter and less useful. A heavier double sestertius was tried briefly around the middle of the century, but the denomination had effectively disappeared by the 270s. Diocletian's reforms later created a new range of base-metal coins, and Constantine's gold solidus reshaped the monetary system that followed.

The legacy of the sestertius is largely literary and administrative. Latin authors expressed everything from senatorial fortunes to the cost of slaves and public buildings in sestertii, so the unit is central to modern studies of Roman prices, wages and inflation. Renaissance collectors prized large sestertii for their portraits, and the coins remain popular with collectors today.

• British Museum: Collection online
• American Numismatic Society