“The standard Roman accounting coin — the sestertius — was named 'two and a half,' from semis (half) and tertius (third), because it was originally worth two and a half asses, and the Romans found it easier to name the coin for its fraction than for its value.”
Sēstertius comes from semis (half) + tertius (third) — literally 'half-third,' meaning 'the third half,' which the Romans used to mean 'two and a half.' The coin was originally worth two and a half asses (the as being the basic Roman bronze coin). The name was a fraction, which is appropriate for a denomination that existed primarily for accounting rather than spending. Romans kept records in sestertii. They spent in denarii.
The sestertius was originally a small silver coin, but under Augustus (27 BCE - 14 CE) it was reissued as a large brass coin — the largest denomination in regular circulation. The brass sestertius became the Roman coin most familiar to modern collectors: large, heavy, with detailed portraits and architectural scenes. The Colosseum, Trajan's Column, and the harbors of Ostia all appear on sestertii. The coin was a portable propaganda medium. Every transaction spread the emperor's image.
Prices in Roman literature are almost always quoted in sestertii. A legionary soldier earned 900 sestertii per year under Augustus. A modest house in Pompeii cost around 30,000 sestertii. Entry to the public baths cost one-quarter of a sestertius. These prices give modern historians a detailed picture of Roman economic life. The sestertius is the unit in which Rome's economy was recorded.
The coin ceased to be minted in the third century CE as Roman currency was debased and reformed. The word survived in historical and numismatic vocabulary. It remains the standard unit in which Roman economic data is discussed. When a historian writes 'the bridge cost 500,000 sestertii,' the reader understands — or is expected to — that this was a substantial sum. The coin that was named for a fraction became the standard of Roman value.
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Sestertius is used in Roman history, numismatics, and economic history. The word appears whenever historians discuss Roman prices, wages, or public expenditure. It is the unit in which Rome's economy is measured.
The coin named for a fraction — two and a half — became the standard of Roman value. The name was mathematics. The function was propaganda. Every sestertius carried the emperor's face into every market, every tavern, every transaction in the empire. The fraction became the face of power.
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