“The Roman gold coin called the solidus — from the Latin for solid or whole — was so reliable that it circulated for seven hundred years without significant change, and its name gave English the word 'soldier,' because soldiers were the people paid in solid gold.”
Solidus in Latin means solid, whole, complete — from the idea of a coin that was solid gold, not debased or alloyed. Emperor Constantine I introduced the solidus around 309 CE as part of his monetary reform, setting its weight at 4.5 grams of pure gold (1/72 of a Roman pound). The solidus replaced the aureus, which had been steadily debased over the previous century. Constantine's solidus was full weight, full purity. Solid.
The solidus was the most stable currency in Western history. It maintained its weight and purity for over seven hundred years — from Constantine in the fourth century through the fall of Constantinople in 1453. The Byzantine Empire, which continued minting the solidus (called nomisma in Greek), used it as the international trade currency of the Mediterranean. Arab, Persian, and Indian merchants accepted it. The solidus was the dollar of the medieval world.
The word solidus gave birth to multiple modern words. 'Soldier' comes from solidus via Old French soudier — a person paid in solidi. 'Sou' was the French descendant of solidus. The abbreviation 's' for shilling comes from solidus. The slash mark (/) used in fractions and web addresses is called a solidus in typographic terminology. The coin's name infiltrated English from multiple directions.
The solidus ended when Constantinople fell to the Ottoman Turks in 1453. The coin that had circulated for seven centuries — longer than any other coin in Western history — died with the empire that minted it. But the word survived in French, Italian, English, and typographic convention. The solid coin is in the soldiers, the shillings, and the slashes.
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Today
Solidus is used in numismatics, Roman and Byzantine history, and typography (the solidus is the name for the slash mark /). The word appears in every discussion of late Roman and Byzantine economics.
Seven hundred years. No other Western coin lasted as long. The solidus was solid — reliable, predictable, trustworthy — and its name said so. Constantine named his coin for the quality he wanted the world to see in it, and the world agreed for seven centuries. The solid coin became the soldier's pay, the typographer's slash, and the longest-running monetary promise in European history.
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