The Académie Française says counting in 20s was customary in the Middle Ages (a system called vigesimal, from the Latin). The system was used by the Celts and Normans, who probably brought it with them to Gaul.
Theories abound that its origin may be linked to the number of fingers and toes that we have.
At the time, it was common to ask for vint et dis (30) of something at the market, or deux vins (40) or trois vins (60). Paris still has the Quinze-Vingts blind hospital, founded by Saint-Louis in 1260 and named after the number of beds available (300).
Trente, quarante, cinquante and soixante started to spread towards the end of the Middle Ages, but bigger numbers (requiring more mental arithmetic) stayed as soixante-dix, quatre-vingts and so on. These became officially recognised by the Académie Française dictionary in the 17th century.
Septante, octante and nonante are also in the dictionary. Some small parts of Savoie and the Midi can be heard using them – and they still featured in the national curriculum in 1945 as an easier way to teach counting.
Septante and nonante are used in Belgium and Switzerland, the Democratic Republic of the Congo, Canada, Rwanda and the Channel Islands. They cannot agree on 80, however. The Belgians and some Swiss (around Geneva) use
quatre-vingts. Other parts of French-speaking Switzerland use huitante.
Language expert Camille Chevalier-Karfis, of French Today, does not blame the language, but the way people learn it. She said: "In my opinion, there are a number of reasons why foreigners don't get French numbers: and they have to do with the way they are taught.
"They over-analyse numbers such as 70, 80, or 90. They think of they way they are written and therefore do the maths - 60-plus-10; four-times-20 - but to a French person, 80 is simply quatre-vingts ('katrevin') and 99 is "quatre-vingt-dix-neuf" ('katrevindizneuf'), not (4x20)+(10+9)."
Unfortunately for students of French, she said there is no easy way to learn French numbers: "Students need to spend more time learning small numbers, in particular the weird ones: 11 to 16. Teachers often go too fast over these, because they are a small chunk stuck in what is then explained like a logical mathematical progression.
"However, in larger numbers, students can come unstuck with these "weird numbers." Most French students I know mistake the numbers "treize" (13) and "seize" (16), and struggle between trois, treize and trente (3, 13, 30) and six, seize and soixante (6, 16, 60).
"Numbers are used in everyday life. French people say them very, very fast.
"You need to know them by heart and it takes work. It's not something you can work out, build in your head like a French sentence. Under 100, the information has to come directly to your brain, as one entity.
"I recommend that students first learn the sound of numbers. Forget about the spelling. Forget about the logic behind 70, 80, 90... Just learn the sounds and link that sound to the number."