French sol
The sol, later called a sou, is the name of a number of different coins, for accounting or payment, dating from Antiquity to today. The name is derived from the solidus. Its longevity of use anchored it in many expressions of the French language. Roman antiquity. The "solidus" is a coin made of 4.5 g of gold, created by emperor Constantine to replace the "aureus". Early Middle Ages. Doing honour to its name, the new currency earns the reputation of unalterability, crossing almost unchanged the decline and fall of the Western Roman Empire, the great invasions and the  creation of throughout Europe; not only was it issued in the Byzantine empire until the 11th century under the name of "nomisma", but the "solidus" was imitated by the barbarian kings, particularly the Merovingians, albeit most often in the form of a "third of a sou" ("tremissis"). Facing a shortage of gold, a new "stabilization" (as devaluations are often called) was introduced by Charlemagne: from then on the "solidus" no longer represents 1/12 of the Roman gold pound but 1/20 of the Carolingian silver pound instead. The "sou" itself is divided into 12 denarii and one "denarius" is worth 10  asses. But for rare exceptions (saint Louis' "gros"), the "denarius" will in practice be the only ones in circulation. Charlemagne's general principle of 12 "denarii" worth one sol and of twenty sols worth one pound is kept with many variants according to the alloy used and the dual metal gold:silver sometimes used for some issues. In fact, only members of the money changers corporation could find their way among the equivalences and the many currencies used in Europe at each period, and therefore were unavoidable for many commercial operations. Late Middle Ages. The name evolves as does the  rest of the language, from Latin to French. "Solidus" becomes "soldus", then "solt" in the 11th century, then "sol" in the 12th century. In the 18th century (Ancien Régime) the spelling of "sol" is adapted to "sou" so as to be closer to the pronunciation that had previously become the norm for several centuries. Abolition and legacy. In 1795, the livre was officially replaced by the franc and the sou became obsolete as an official currency division. Nevertheless, the term ""sou"" survived as a slang term for 1/20 of a franc. Thus the large bronze 5-centime coin was called  sou"" (for example in Balzac or Victor Hugo), the ""pièce de cent sous"" ("hundred sous coin") meant five francs and was also called ""écu"" (as in Zola's "Germinal"). The last 5-centime coin, remote souvenir inherited from the "franc germinal", was removed from circulation in the 1940s, but the word "sou" keeps being used (except for the 1960 new franc's five-centime coin which was worth five old francs). Section::::"Sous" outside France. Canada. In Canada, the word "sou" is used in everyday language and means the 1/100th division of the Canadian dollar. The official term is "cent". Canadian  one-cent coins (no longer in circulation) have the vernacular name of ""sou noir"" ("black sou") in Canadian French. The Canadian quarter, valued at 25 cents, is called "trente sous" ("thirty sous"). This usage dates from when the word “"sou"” was used in French-speaking Lower Canada to refer to the halfpenny coin of the Canadian pound; at that time an American quarter was valued at 1 shilling 3 pence Canadian (i.e. 15 pence Canadian), and the usage remained after Canada switched currencies. ""Échanger quatre trente sous pour une piastre"" ("to exchange four 30 sous for one piastre") therefore means  changing something for an identical thing, as the "piastre" is the common name for the Canadian dollar. Switzerland. In Switzerland, a hundred-sou coin is a five-Swiss-franc coin and a four-sou coin is a twenty-Swiss-centime coin. The word sou also remains in informal language in the terms "ten, twenty ... sous". Vietnam. The French term "sou" was borrowed into Vietnamese as the word "xu" / "su" (樞). The term is usually used to simply mean the word "coin" often in compound in the forms of "đồng xu" (銅樞) or "tiền xu" (錢樞). The modern Vietnamese đồng is  nominally divided into 100 "xu". The sou in French expressions. Used for over a thousand years, the word ""sou"" is deeply rooted in the French language and expressions. "Les sous", plural, is a synonym for money. « "Se faire des sous" », to make money.