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Roman and Byzantine coinage 
The solidus was introduced by Constantine I in 312 AD, permanently replacing the aureus as the gold coin of the Roman Empire. The solidus was struck at a rate of 72 to a Roman pound of pure gold, each coin weighing twenty-four Greco-Roman carats, or about 4.5 grams of gold per coin. By this time, the solidus was worth 275,000 increasingly debased denarii.
The solidus was maintained essentially unaltered in weight and purity until the 10th century, though in the Greek-speaking world during the Roman period and then in the Byzantine economy it was known as the nomisma (plural nomismata). Whenever the coin was taken in by the treasury, it was melted down and reissued. This maintained the evenness of the weight of the circulating solidi, since the coin did not tend to be in circulation long enough to become worn.
Minting of the gold coin—unlike the base-metal coins of the time—had no permanently established minting facility. Due to the requirement that taxes be paid in gold, solidus minting operations tended to follow the emperor and his court. For example, solidi were minted in Milan in 353, and in Ravenna after 402. These locations were imperial residences at the respective times.
Although merchants were forbidden to use solidi outside the Byzantine Empire, there was sufficient trade in these coins outside the empire that they became a desirable circulating currency in Arabian countries. Since the solidi circulating outside the empire were not used to pay taxes to the emperor they did not get reminted, and the soft pure-gold coins quickly became worn.
Through the end of the 7th century, Arabic copies of solidi—dinars minted by the caliph Abd al-Malik ibn Marwan, who had access to supplies of gold from the upper Nile—began to circulate in areas outside the Byzantine Empire. These corresponded in weight to only 20 carats (4.0 g), but matched the weight of the worn solidi that were circulating in those areas. The two coins circulated together in these areas for a time.
Except in special cases, the solidus was not marked with any face value throughout its seven-century manufacture and circulation. Solidi were wider and thinner than the Aureus, with the exception of some lower-quality issues from the Byzantine Empire. Fractions of the solidus known as semissis (half-solidi) and tremissis (one-third solidi) were also produced.
The word soldier is ultimately derived from solidus, referring to the solidi with which soldiers were paid.
Impact on world currencies 
In medieval Europe, when the only coin in circulation was the silver penny (denier), the solidus was used as a unit of account equal to 12 denier. Variations on the word solidus in the local language gave rise to a number of currency units:
In the French language, which evolved directly from common or vulgar Latin over the centuries, solidus mutated to soldus, then solt, then sol and finally sou. No gold solidi were minted after the Carolingians adopted the silver standard; thenceforward the solidus or sol was a paper accounting unit equivalent to one-twentieth of a pound (librum or livre) of silver and divided into 12 denarii or deniers. The monetary unit disappeared with decimalisation and introduction of the Franc during the French revolution (1st republic) in 1795, but 5 centimes, the twentieth part of the Franc, inherited the name as a nickname.
To this day, in French around the world, solde means the balance of an account or invoice, and is the specific name of a soldier's salary. Sou is also used as slang for a small coin of little value, as in sans le sou. "I'm broke", "without money".
In French-speaking Canada sou is a commonly employed term for the Canadian cent (standard French, centime, not used in French-speaking Canada) although the term "cent" (colloquially pronounced cenne ("sen") when referring to currency in French-speaking Canada) is also widely used. Interestingly, quarter dollars in Quebec French are often called trente sous (thirty cents) because of a series of changes in terminology, currencies, and exchange rates. After the British conquest of Canada in 1759, French coins gradually went out of use, and sou became a nickname for the halfpenny, which was similar in value to the French sou. Spanish dollars and U.S. dollars were also in use, and from 1841 to 1858 the exchange rate was fixed at $4 = £1 (or 400¢ = 240d). This made 25¢ equal to 15d, or 30 halfpence (trente sous). In 1858, pounds, shillings, and pence were abolished in favour of dollars and cents, and the nickname sou began to be used for the 1¢ coin, but the idiom trente sous for 25¢ endured.
In Romanian, sold means the balance of an account or invoice, while soldă means the payment for military services.
The name of the medieval Italian silver soldo (plural soldi), coined since the 11th century, was derived from solidus.
This word is still in common use today in Italy in its plural soldi with the same meaning as the English equivalent "money".
Spain and Peru, Portugal and Brazil 
As with soldier in English, the Spanish and Portuguese equivalent is soldado (almost the same pronunciation). The name of the medieval Spanish sueldo and Portuguese soldo (which also means salary) was derived from solidus, which is also used in the Philippines as Suweldo.
The Portuguese word saldo, as the forementioned French solde, also means the balance of an account or invoice. And, for that matter in Afrikaans the word "saldo" means the balance of an account.
Some have suggested that the Peruvian unit of currency, the sol is derived from solidus, but the standard unit of Peruvian currency was the real up until 1863. Throughout the Spanish world the dollar equivalent was 8 reales ("pieces of eight"), which circulated legally in the United States until 1857. We hear echoes of that time in the expression "two bits" for a quarter dollar, and the real was last used for accounting in the US stock market, which traded in 1/8 dollars until 2001.
The Peruvian sol was introduced at a rate of 5.25 per British Pound, or just under four shillings (the legacy soldus). The term soles de oro was introduced in 1933, three years after Peru had actually abandoned the gold standard. In 1985 the Peruvian sol was replaced at one thousand to one by the inti, representing the sun god of the Incas. By 1991 it had to be replaced with a new sol at a million to one, after which it remained reasonably stable.
United Kingdom 
King Offa of Mercia adopted the Frankish silver standard of libra, solidi and denarii; although for centuries English coinage was restricted to the silver penny the scilling, understood to be the value of a cow in Kent or a sheep elsewhere, was a 12d money of account like its French equivalent, until the Tudors minted the first shilling coins. Prior to decimalisation in the United Kingdom in 1971, the abbreviation s., from solidus, was used to represent shillings, just as d. and £, from denarius and Librum, were respectively used to represent pence and pounds, leading to the abbreviation "£sd." The common use of a slash, e.g. 2/6 for two shillings and sixpence, was a holdover of the old orthographic long S.
See also 
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- Roman currency
- Byzantine coinage
- Hoxne Hoard
- Republic of Corsica
- Solidus (punctuation)
- Slash (punctuation) (also called a solidus)
- Porteous 1969
- "Merriam-Webster Online Dictionary". Merriam-Webster. Retrieved 2008-12-05.
- Spufford, Peter (1993). Money and its use in medieval Europe. Cambridge University Press. pp. 7–9,18–19,25,33–35,37,50–52,397,400. ISBN 978-0-521-37590-0.
- Frédéric Farid (26 September 2008). "Pourquoi trente sous = 25 cents ?". Retrieved 6 October 2010.
- Hodgkin, Thomas (1906). The history of England ... to the Norman conquest. London, New York and Bombay: Longmans, Green, and Co. p. 234.
- Porteous, John (1969). "The Imperial Foundations". Coins in history : a survey of coinage from the reform of Diocletian to the Latin Monetary Union. Weidenfeld and Nicolson. pp. 14–33. ISBN 0-297-17854-7.
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- Online numismatic exhibit: "This round gold is but the image of the rounder globe" (H.Melville). The charm of gold in ancient coinage