A solidus (the Latin word for solid) was originally a gold coin issued by the Romans. It was introduced by Constantine I in the AD 309–10, and was used through the Byzantine Empire until the 10th century AD. The coin replaced the aureus as the main gold coin of the Roman Empire.
The name solidus had previously been used by Diocletian (284-305) for the gold coin that he introduced, which is different from the solidus introduced by Constantine. The coin was struck at a theoretical value of 1/72 of a Roman pound (about 4.5 grams). Solidi were wider and thinner than the aureus, with the exception of some dumpy issues from the Byzantine Empire. The weight and fineness of the solidus remained relatively constant throughout its long production, with few exceptions. Fractions of the solidus known as semisses (half-solidi) and tremissis (one-third solidi) were also produced.
Impact on world currencies
Variations on the word solidus gave rise to a number of currency units:
The current currency of Peru is the sol. This was often interpreted as sol meaning "sun", and during the monetary reforms of 1985 it was replaced with the inti, from the Quechua word for sun. The sol was reintroduced as the nuevo sol in a subsequent reform.
In France the sou (until 1715 sol) was the name of a coin. It was first minted in gold, from the 1200s in sliver and during the 1700s in copper. The sou tournois was a 12-denier coin, one-twentieth of the livre tournois (Tournois pound), while the sou parisis was a 15-denier coin. After decimalization in France, the sou became the name for a five-centime coin, one-twentieth of the French franc.
To this day, sou is used as slang for money, as in j'ai pas de sous. "I'm broke", "I haven't got two bob to rub together".
Until decimalization in the United Kingdom in 1971, the abbreviation s., from solidus, was used to represent a shilling, worth one-twentieth of a Pound Sterling, just as d. stood for denarii (pennies) and £ stood and still stands for Libra (pound). Though the shilling and its abbreviation are no longer used, a slang word for shilling, "bob", is still occasionally used in expressions like "a few bob", i.e. a bit of money.