309: Constantine's gold solidus

Around 309 AD, Constantine the Great introduced a new gold coin, the solidus, struck at 72 to the Roman pound – about 4.5 grams of gold. After a century of inflation and debased silver, it restored trust in Roman money.

The solidus kept its weight and purity in the Byzantine Empire for roughly seven centuries and is sometimes called the 'dollar of the Middle Ages'. Its name survives in the Italian 'soldo', the French 'sou' and the word 'soldier' – someone paid in solidi.

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