AS, the Roman unit of weight and measure, divided into 12 unciae (whence both "ounce" and "inch"); its fractions being deunx 11/12, dextrans 5/6, dodrans , bes 2/3, septunx 7/12, semis , quincunx 5/12, triens 1/3, quadrans , sextans 1/6, sescuncia 1/8, uncia 1/12. As really denoted any integer or whole; whence the English word "ace." The unit or as of weight was the libra (pound: = about 114/5 oz. avoirdupois); of length, pes (foot: = about 113/5 in.); of surface, jugerum (= about 2/3 acre); of measure, liquid amphora (about 53/5 gal.), dry modius (about 9/10 peck). In the same way as signified a whole inheritance; whence heres ex asse, the heir to the whole estate, heres ex semisse, heir to half the estate. It was also used in the calculation of rates of interest.
As was also the name of a Roman coin, which was of different weight and value at different periods (see Numismatics, § Roman). The first introduction of coined money is ascribed to Servius Tullius. The old as was composed of the mixed metal aes, an alloy of copper, tin and lead, and was called as libralis, because it nominally weighed 1 lb or 12 ounces (actually 10). Its original shape seems to have been an irregular oblong bar, which was stamped with the figure of a sheep, ox or sow. This, as well as the word pecunia for money (pecus, cattle), indicates the fact of cattle having been the earliest Italian medium of exchange. The value was indicated by little points or globules, or other marks. After the round shape was introduced, the one side was always inscribed with the figure of a ship's prow, and the other with the double head of Janus. The subdivisions of the as had also the ship's prow on one side, and on the other the head of some deity. The First Punic War having exhausted the treasury, the as was reduced to 2 oz. In the Second Punic War it was again reduced to half this weight, viz. to 1 oz. And lastly, by the Papirian law (89 B.C.) it was further reduced to the diminutive weight of half an ounce. It appears to have been still more reduced under Octavian, Lepidus and Antony, when its value was 1/3 of an ounce. Before silver coinage was introduced (269 B.C.) the value of the as was about 6d., in the time of Cicero less than a halfpenny. In the time of the emperor Severus it was again lowered to about 5/24 of an ounce. During the commonwealth and empire aes grave was used to denote the old as in contradistinction to the existing depreciated coin; while aes rude was applied to the original oblong coinage of primitive times.
Note - this article incorporates content from Encyclopaedia Britannica, Eleventh Edition, (1910-1911)