Solidus

The solidus was the gold coin introduced by the Roman emperor Constantine I in the early fourth century CE and struck by his successors in the eastern Mediterranean for some seven centuries. Weighing about 4.5 grams and made of nearly pure gold, it kept its standard with remarkable consistency until the eleventh century and was trusted far beyond the borders of the empire. In Greek, the language of the Byzantine state, the same coin was called the nomisma.

The Latin name means "solid" or "whole". Medieval western Europeans knew Byzantine gold as the bezant, and the solidus survived as a unit of reckoning behind the "s." abbreviation for the shilling, the French sou and the Italian soldo.

By the early fourth century the Roman aureus had been struck at shifting weights for decades. Around 309–310 CE Constantine began issuing a new gold coin at 72 to the Roman pound, first in the western territories he controlled and, after his victory over Licinius in 324, across the whole empire. Ancient writers report that gold confiscated from pagan temples helped supply the mints. Taxes were increasingly assessed and paid in gold, and the solidus became the basis of state finance, army pay and large private transactions. Its fractions, the semissis (a half) and the tremissis (a third), circulated alongside it, and for accounting it was divided into 24 siliquae, called keratia in Greek.

Nomisma is simply the ordinary Greek word for a coin, related to nomos, meaning law or custom. As Greek replaced Latin as the language of government in the eastern empire, the solidus was routinely called the nomisma, so the two names describe the same coin rather than two different currencies. Byzantine documents used the nomisma as the main unit of account for taxes, salaries and prices.

Justinian I (527–565) struck gold solidi in large numbers to pay for building programmes, wars in North Africa and Italy, and payments to neighbouring powers. His solidi carry his armoured bust on the obverse and, on the reverse, a winged figure holding a cross, part of a broader shift towards Christian imagery. Under Justinian II, at the end of the seventh century, the bust of Christ appeared on the obverse of the solidus for the first time.

The coin's prestige shaped neighbouring currencies. Early Islamic rulers first adapted Byzantine designs in Arab-Byzantine coinage before introducing the purely inscribed Umayyad dinar in the 690s, at a weight slightly below that of the solidus. In the 960s Nikephoros II Phokas added a lighter gold coin, the tetarteron, alongside the full-weight coin, which became known as the histamenon.

From the reign of Michael IV (1034–1041) the gold content began to fall. The early reductions were modest, but under pressure from wars, lost territory and court spending they accelerated sharply in the 1070s and 1080s, when some issues contained only a fraction of their former gold. This is one of the best-documented cases of currency debasement in medieval history. It left the empire with a confusing mix of coins of different fineness, a situation in which Gresham's law favoured the circulation of the worst coins.

In 1092 Alexios I Komnenos replaced the debased issues with a new gold coin, the hyperpyron, meaning "highly refined". It was struck at roughly 20.5 carats, a little below the old standard, and like other coins of the period it had a shallow cup shape. The hyperpyron remained the Byzantine gold coin until the mid-fourteenth century, after which Byzantine gold coinage ended and the empire relied on silver.

For several centuries the solidus was the most widely trusted gold coin in the Mediterranean and western Asia, and historians have compared its role to that of a modern reserve currency. Its long stability made it a benchmark for the gold coins that took over its commercial role in the thirteenth and fourteenth centuries, notably the Florentine florin and the Venetian ducat.

In the West the solidus also became a unit of account linked to silver. Under the Carolingian system, twelve deniers, the descendants of the Roman denarius, made one solidus, and twenty solidi made one pound. That arrangement survived in British pounds, shillings and pence until 1971.

• British Museum: Collection online
• American Numismatic Society