The term 'nummus' (coin) in Roman literature usually refers to the sesterce – the smallest silver denomination of the Roman Republic, which was minted only sporadically and in small quantities. Nevertheless the sesterce became the unit of account of the Roman Republic. Even at the time of the Roman Empire large sums, assets, taxes and so on, were still quoted in sesterces.
The sesterce was introduced in the course of a reorganization of the Roman currency around 211 BC. The obverse featured the head of the goddess Roma with an Attic helmet and the specification of value, IIS. The reverse depicted the Dioscuri Castor and Pollux and the declaration of the coin's origin, ROMA.