"Sestertius" is actually a contraction of the Latin "SEMIS TERTIVS" which meant 2˝. The sestertius as a coin has its origins in the great Republican coinage reform of 211 BC. The silver "denarius"was instituted about 211 BC and had a value of ten of the new reduced asses (see As) struck to the 'Sextantal' standard of about 48-55 grams each (the weight of an early heavy sextans). The silver sestertius was created to be a quarter of the denarius, which meant it was worth 2˝ asses (hence its name). Since the denarius originally weighted four scripuli (about 4.52 grams), its quarter fraction (sestertius) was struck at one scripulum, or 1.13 grams. The first sestertii carried the value marking "IIS" which meant two and a 'semis' (half) asses.
Production of silver sestertii ceased by the middle of the second century BC when the denarius had fallen to about 3˝ scripuli (3.96 grams). When production of silver sestertii was resumed early in the first century BC, its weight was proportionately reduced to about 0.98 grams. Another development during that time period was the further reduction of the weight of the bronze coins to the point that a denarius was now worth 16 asses instead of the original 10. Again the silver sestertius followed the denarius and assumed a value of 4 asses, which is what it was valued at when the Imperatorial era began. Marc Antony began to issue the sestertius as a bronze coin with dual value markings: a Greek "delta" (4) to show it was worth 4 asses, and "IIS" with a line through the middle which had become the accepted symbol for the sestertius. (The "IIS" with a line through the middle eventually became simply "HS" in Roman notation.)
The sestertius became an orichalcum coin for the remainder of its existence under the Empire. Augustus (27 BC-14 AD) struck the coins in orichalcum at about 25 grams, although since it was no longer a precious metal the weights could easily vary ten per cent. Orichalcum was a brass alloy which in the first century was comprised of about 80% copper and 20% zinc. In the second century the alloy was still about 80% copper, but the zinc content fell to only about 5%, with the rest being composed of tin and lead. The early Imperial sestertii were very large and impressive coins, with first century AD average weights between 25 and 27 grams and diameters between 33 and 38 mm's. The sestertii of the first century are often considered to be among the most beautiful coins the Empire struck. The second century saw rather stable weights and diameters of about 25 grams and 34 mm's, respectively. The third century saw quite a decline in the sestertius as it shrank to about 20 grams and a diameter of 25-30 mm's by the reign of Severus Alexander (225-238).
The sestertius, like all bronze coinage, was doomed by the introduction and subsequent debasing of the"antoninianus." By the reign of Gallienus (253-268) the antoninianus' weight had diminished to less than 3.5 grams and its silver content had decreased to 4%, although it was still officially equal to two denarii (eight sestertii). The preposterous situation where an essentially bronze coin of 3.5 grams was made equal to eight bronze coins of 20 grams each (160 grams total) soon made the government unwilling to strike sestertii and made the people unwilling to circulate them. The result was that the last sestertii were struck sometime in the reign of Postumus, the Gallic usurper (260-268) who attempted not only to preserve the sestertius but actually attempted to revive its double (which had been struck only under Trajan Decius (249-251).
The sestertius is sometimes referred to in older literature as a "first bronze", while its orichalcum half, the dupondius, is referred to as a "second bronze."
Hoffman, quoted by Rasche, says - "Four sesterces make a denarius, that is ten asses, which, if it is silver, is equal in weight to a drachm."
The sesterce has for its types, on one side a female head helmeted and winged, behind it IIS., on the reverse are the Dioscuri on horseback, and below ROMA. - This little coin is by no means common. Eckhel had seen but two; one belonging to the Cordia family, ascertained to be a sesterce solely by its weight; the other to the Sepullia family, which, besides the right weight, had the mark
The simple sesterce, or little sesterce, says Kolb, was worth about four sous French money (2d. English).
At the epocha when, according to the generally received opinion, silver money was introduced at Rome, viz., in the year 269 before Christ (485th of the city), the monetal unit (l'unité monétaire) was changed; the As, which had become successively of a less important value, ceased to be used in numbering sums. The sesterce was adopted as the monetal unit, probably because this real money (monnaie effective) was the intermedial coin of three established forms of specie.