Idiosyncrasies of French-language numeracy

True numeracy requires that you be able to understand numbers, said or written as words, saying them or writing them as words.  Here are some idiosyncrasies, which often fool speakers of American English.

Pronunciation: 6 and 10 have three pronunciations. Any number ending in a consonant has at least two pronunciations. This means it could be hard to distinguish between
"deux", "trois" or "six étudiants" and "douze", "treize" or "seize étudiants".

Teentalk: thirteen, fourteen, ... nineteen.  We have seven numbers with the syllable "teen" in them. If you accept the prefix "...dix" as the equivalent of "teen", then it gets crazy.  In French, we have "dix-sept, dix-huit, dix-neuf", but don't confound these with the numerical suffix "dix" in "soixante-dix." and "quatre-vingt-dix". Evenything from 11-16 ends in a "eze" sound. What is more, you wind up saying the numbers from "onze" through "dix-neuf" three times in every slice of one humdred.  The reason for this is another ideosyncracy. French 70 is "soixante-dix", and subequent numbers "soixante et onze, soixante-douze...". 90 is "quatre-vingt-dix" with subsequent numbers "quatre-vingt-onze, quatre-vingt-douze..." Don't discount the imprtance of ten.  Sequences of ten dominate and make logical our counting in English. We count from one to nine orally nine times on our way to one hundred (9, 29-99), and ten times in digits and written verbal representation (+19).  This still happens six times in French (19-69).

We're number one! Numbers containing the digit 1 at the end are subect to another ideosyncracy. Here they are from 1-101 "un, onze, vingt-et-un, trente-et-un, quarante-et-un, cinquante et un, soixante et un, soixante et onze, quatre vingt un, quatre-vingt-onze, cent un". Note the appearance and disappearance of the word "et". Note its conflation with another idiosyncrasy: sixty-ten (=soixante-dix). sixty and eleven (soixante et onze).

The use of plural "s" is somewhat of a mystery. The French write 80 as "quatre-vingts", but 81 as "quatre-vingt-un". 200 is "deux cents", while 201 is deux cent un". There is no issue with 2000, because "mille" does not take as "s" in the plural. However 2,000,000 is "deux millions", which unlike the hundreds, retains its "s" in subequent numbers ("deux millions trente mille..."). Where this is a numerical adjective it will be "deux millions de...".

In Switzerland and Belgium, there are many who use the logical numeracy of "septante" for 70, "huitante" for "80" and "nonante" for 90. The French system for these numbers may stem from remarks of 17th-centery grammarians Vaugelas and Menage. This means that nearly 30% of their numbers are differente from those in the rest of the French-speaking world, and that they do not repeat the numbers from 11-19 three times on the way from 1 to 100. Of course this is minority and therefore dialectal discourse.

In ordinal numbers, which are numerical adjectives, there are four spellings for "first": "permier, première, premiers, premières."  In dates, only the first of the month can be ordinal ("le premier mai", but "le deux mai"). Ordinality for centuries is often expressed using Ronam numerals ("le XXIe siècle").

Basic math ("le calcul") terminology variations may reflect degrees of formality:

Moreformal Less formal Familiar
2 plus 2 égale 4 2 et 2 font 4 2 et 2, ça fait 4
2-2=0 2 moins 2 égale 0 2 moins 2 font 0 moins 2, ça fait 0
2×2=4 2 multiplié par 2 égale 4 2 fois 2 font 4 2 fois 2, ça fait 4
2÷2=1 2 divisé par 2 égale 1 2 divisé par 2 font 1 2 sur 2, ça fait 1

Accountant's nightmare:  In France, often there is an exchange between the comma and the decimal point (11,235 may be "onze et deux cent trente-cinq milliers", rather than "eleven thousand two hundred thirty five". 11.235 will probbly be "onze mille deux cent trente cinq" rather than eleven and two hundred thirty-five one thousands.

hyphenation: between 1 and 100, two-digit numbers with "et" in them (21, 31, 41, 51, 61, 71) are not hyphenated (vingt et un, etc.), but the rest are. This is also true for the same used in multi-digit numbers.

Numbers beyond us:     
                English            French
                a billion            un milliard de
                a trillion            un billion de, mille millards de