For the majority of the time it was in use, the sestertius was the largest base-metal coin used in the Roman Empire.
It was introduced by the emperor Augustus in or about 23 BC at a value of a quarter of a denarius, and remained the largest base metal denomination in the Roman empire until late in the 3rd century AD.
Caligula Obverse Caligula Reverse
The name sestertius derives from the Roman words semis (half) and tertius (third), in which (third) refers to a third as - the sestertius was equivalent to two full asses and half a third.
The denomination sestertius was used as a standard unit of account throughout that period of time - expenses and income expressed in hundreds, thousands or millions of sestertii.
The purchasing power of the sestertius obviously changed over the centuries - at around the time of the disaster at Pompeii a sestertius would buy half a litre of wine, while a few decades later, Roman soldiers received around 1 - 2 sestertii per day in wages after expenses.
The sestertius was favoured by coin engravers as it allowed them a larger surface area across which they could portray their designs, and despite their relatively low face value, they are preferred by many collectors today for that very same reason.