From shells to the solidus: a short history of money

Tina types a date as 6/10/2026 without a thought for the slash. Yet that slash has an official name, solidus, and it leads back to a gold coin made about 1,700 years ago. Why did people trust that coin for 700 years, when other money lost their trust within a few years?

Before coins, people weighed metal

The previous lesson showed that good money is durable, portable, divisible and widely accepted. Early communities used shells, salt, cattle and lumps of metal. Money that is also a useful good in itself is called commodity money.

Metal worked best, but at every sale someone had to weigh it and check that it was not mixed with something cheaper. That took time and trust.

The first coins: a stamp you can trust

Some of the oldest known coins come from Lydia, in today’s western Turkey, from about 650 BC. They were made of electrum, a natural mix of gold and silver. Each piece followed a fixed weight standard and carried a lion’s head, probably a royal symbol.

The stamp was a promise: “this piece has a known weight”. Buyers and sellers could count coins instead of weighing metal. The idea soon spread to the Greek world and beyond.

When rulers cheat: debasement

A stamp is only as honest as the ruler behind it. Mixing cheaper metal into coins while keeping the same face value is called debasement. It lets a ruler make more coins from the same gold or silver, and spend them.

This is what happened in Rome. In the 200s AD, emperors kept cutting the silver in their coins until only a few percent was left. Prices rose sharply. A general rise in prices is called inflation.

People also noticed which coins were better. When an old, pure coin and a new, weaker coin have the same face value, people spend the weak one and keep the good one at home. This pattern is called Gresham’s law: “bad money drives out good”.

The solidus: 700 years of trusted money

Around AD 309–312, the emperor Constantine introduced a new gold coin, the solidus, Latin for “solid”. It was struck at 72 coins to one Roman pound of gold, about 326.6 g. So each coin held about 326.6 ÷ 72 = 4.5 g of almost pure gold.

What made it special was that it barely changed. In the eastern Roman (Byzantine) Empire, its weight and purity stayed nearly the same for more than 700 years. Real debasement began only in the 11th century, and the coin was replaced in 1092. Byzantine gold coins have been found as far away as China.

The coin lives on in words. A soldier was originally someone paid in solidi, and the table below shows more of its descendants. The word saldo (account balance) is a cousin rather than a child: it comes from the same Latin word, meaning “solid” or “settled”, not from the coin. Salary is not related; it comes from a different Latin word, salarium.

Even the slash carries the name. Until 1971, British money used pounds, shillings and pence, written £sd from the Latin librae, solidi, denarii. There were 12 pence in a shilling and 20 shillings in a pound. The shilling was the solidus, so “2/6” meant two shillings and sixpence. The stroke between them became known as a solidus, and SOLIDUS is still the official computer-standard name for ”/”.

This course is named after the solidus: money people could trust because nobody secretly changed it.

Paper money began in China

In Sichuan, in China, people paid with iron coins that were heavy and low in value. Buying a roll of silk could take tens of kilograms of coins. In the early 11th century, merchants began issuing paper notes, called jiaozi, that stood for coins kept in deposit. In the 1020s the Song government took over and issued the world’s first government paper money.

Paper solved the weight problem but created a temptation: printing notes costs almost nothing. The government issued more and more, the notes lost value, and in 1105 they were replaced. Too many coins in Rome and too many notes in China had the same result: rising prices.