Aes, bronze, also more loosely copper or brass, hence (a) money, coinage, pay, period for which pay is due, campaign; (b) document on bronze. The earliest Roman monetary system involved the weighing out of bronze by the pound or its fractions (see weights); transactions per aes et libram, by bronze and balance, became fossilized in Roman private law as a formal means of transferring ownership. Sums recorded in the sources as ‘so many aeris (gravis, heavy, or rudis, raw)’ are to be taken as intended to mean so many pounds of bronze and then so many coins weighing (more or less) a pound and called asses. In the late 140s bce, the Roman state changed from reckoning in asses to reckoning in sestertii = ¼ denarius = 4 asses each; at this point certain valuations were probably transferred from asses to the same number of sestertii, although the real value was different.
Aes alienum = debt; aes equestre = money assigned to an eques for the purchase of a horse; aes hordiarium = annual sum assigned to an eques for the maintenance of his horse. (Aes signatum in antiquity meant ordinary struck coinage, not, as some Italians suppose, the early Roman (and Italian) currency bars which have been misleadingly called aes signatum by numismatists from the 19th cent. onwards.)