Any French learner will agree that one of the trickiest things to get to grips with is the French number system. It might be smooth sailing until you get to 69, but then funny things start to happen because French from France don't have a separate word for seventy, or eighty, or even ninety for that matter.
70 is soixante-dix (sixty and ten)
80 is quatre-vingts (four times twenty)
90 is quatre-vingt-dix (four times twenty and ten)
Quatrevingt-dix-neuf is a beautiful, melodious number that rolls off the back of the tongue and delights the ear. It also makes no sense. Four twenties and nineteen? Seriously? If you're a salesman trying to sell out discounts TVs for €99.99 in France, you might be tempted to move your business to Belgium - and simplify the math in the process. There you will hear:
70 is septante (from 7, sept)
90 is nonante (from 9, neuf)
Curiously, 80 is still quatre-vingts in Belgium - wouldn't be as much fun otherwise, would it? You'll have to move to Switzerland to replace it with a more practical huitante. But then, you'll have to switch currency.
Why these crazy numbers?
How did the French end up with that bizarre numbering system, while Belgians - with a little trap at 80 - and Swiss, didn't? The reason, as always, is historical.
Whereas the Romans used a decimal system, the Celts used a vigesimal numeral system, based on multiples of 20 - instead of 10. It is believed that civilizations that used vigesimal numbers, such as Mayans, Aztecs, Vikings, Gaelic and Ainus to only name a few, counted with both hands AND feet. Fingers and toes included, you get 20.
When the Roman invaders came to Gaul, they imposed their language and methods on the inhabitants. But they were never entirely successful with their numbering system. There was a short period during the early Middle Ages when the roman versions ‘septante’, ‘huitante’ and ‘nonante’ looked like they were going to stick. And they did in what is today Belgium, Switzerland and a significant chunk of Eastern and Southern France. Other regions, however, resisted change, particularly with the high numbers. In the 17th century, the Académie Française ruled in favor of a mixed system where numbers would be decimal up to 69, and vigesimal from 70 to 100. Belgium and Switzerland, on the other hand, let people decide for themselves. Unsurprisingly, people gradually went for a more consistent system all the way to 100.
Why did the Académie Française ditch the roman logic halfway through? Well, no one knows for sure, but it is believed that usage played an important part. At the time, the lower numbers were used daily by a larger population that had adopted the decimal system early on, because, admittedly, it made life easier. However the larger numbers were more often used by scientists and professionals who had trained in the vigesimal system and held fast to it.
This is the story of the seemingly schizophrenic numbering system in France.