Sou

The sou was a French money unit worth one-twentieth of a livre, or twelve deniers. Its name comes from the late Roman solidus, and for most of its history it was a money of account rather than a coin: prices, wages and taxes were reckoned in sous long before a single piece bearing that value was struck.

The unit outlived the old monarchy. When France adopted the decimal franc at the end of the eighteenth century, the word attached itself to the five-centime coin, and it survives today in everyday French expressions about money and poverty.

The sou belongs to the reckoning system set up under the Carolingian rulers in the late eighth century. One pound of silver (the livre) was divided into 20 solidi, and each solidus into 12 silver pennies (the denier, from the Roman denarius). Only the denier was actually minted. The livre and the sou were counting units, a way of expressing large sums in multiples of a small silver coin.

The same 1 : 20 : 240 structure spread across much of western Europe. In England it became pounds, shillings and pence, the system behind the English penny and the pre-decimal pound sterling, where the abbreviation "s." for shilling still pointed back to the solidus. Italy had the soldo and the Iberian kingdoms the sueldo.

For centuries the sou functioned mainly as a unit of account. Medieval France used more than one standard: the livre tournois, based on the money of Tours, and the heavier livre parisis of Paris, so a "sou tournois" and a "sou parisis" were different amounts. The tournois standard gradually prevailed, and the Paris reckoning was abolished in the later seventeenth century.

Because the denier was repeatedly reduced in silver content, the real metal value behind a sou fell steadily over the medieval and early modern period. This long currency debasement meant that the sou, which had once stood for a substantial sum, became a small everyday amount. Gold and large silver coins such as the écu and the louis d'or circulated alongside the accounting units, and their official values in livres, sous and deniers were changed by royal decree from time to time.

Coins worth a sou did eventually appear. The gros tournois introduced under Louis IX in the 1260s was valued at twelve deniers, making it in effect the first sou coin, although its value in account money later rose. By the seventeenth and eighteenth centuries base-metal pieces denominated in sols (the older spelling) and their multiples were part of ordinary small change, and copper sols were still being struck in the early years of the Revolution.

The revolutionary governments replaced the livre with the decimal franc in the 1790s, and the franc of 1803 fixed its metal content within a bimetallic system of gold and silver. The new French franc was divided into 100 centimes, and the old livre and franc were close enough in value that the five-centime piece, one-twentieth of a franc, inherited the name sou. A five-franc coin was commonly called "cent sous".

Official accounts no longer used the sou, but shopkeepers and the public kept it alive for well over a century. Price rises in the twentieth century, culminating in the revaluation of 1960 that created the new franc, gradually made the five-centime coin almost worthless and the term faded from daily pricing.

The word remains common in French idiom. "Être sans le sou" means to be penniless, "ne pas avoir un sou" to have no money at all, and "une machine à sous" is a slot machine. "Un sou est un sou" expresses thrift, much as English speakers talk about pennies. In Canadian French, "sou" is still widely used for the cent of the Canadian dollar, and the word continued in casual French speech after the franc gave way to the euro in 2002.

The story of the sou illustrates how an accounting unit can outlast the coins and systems that produced it. Its path from a Roman gold coin to a medieval reckoning unit, then to a small decimal coin and finally to a figure of speech, follows the wider erosion of value caused by long-run inflation and repeated monetary reform.

• British Museum collection
• American Numismatic Society
• Banque de France