Why isn’t there a French word for 80?

Anyone learning to count in French will have noticed that something strange happens when you reach 80. The word for 80 isn’t octante - unless you’re Belgian, or Swiss (more on that later) - but quatre-vingts, which literally means 'four twenties'. Nor is there a word for ninety; instead you have to say quatre-vingt dix. More complex numbers, for instance 84 and 92 - quatre vingt quatre and quatre vingt douze respectively - are especially unwieldy.

But why is this the case? As so often with quirks in the French language, you have to go back quite a while to see the reason behind this. To understand why French numbers are the way they are, however, we don't need to go back to early French, or even Latin, but to a language that was spoken in modern-day France from between the 5th century BC to the 5th century AD: Gaulish.

The Gauls did not count numbers the same way we do. They used what is called a vigesimal system, where multiples of twenty are used as a base for building more complex numbers. English, by contrast, is a Base-10, or decimal, system.

The Gauls were conquered by the Romans in the 1st century BC, although it took a few centuries for the people of what is now France to fully adopt - and adapt - the Latin language, eventually turning it into the language known as French. As they adopted the Romans' language, they also adopted the Romans’ decimal system for numbers up to sixty. That’s why we say 'cinquante' in French, instead of 'deux-vingts-dix', for instance.

How come the decimal system stops at 70 in French? Linguists don’t really know, but it could be due to the fact that eighty is simply a rarer number than sixty, forty, and so on. Saying ‘deux-vingts’ or ‘trois-vingts’ on a regular basis would probably have required more mental gymnastics than the average medieval clerk would care to handle, compelling speakers of early French to use the more logical formulation. When the Académie Française came to standardise the language in the 17th century, 'quatre-vingts', 'quoter-vingts-une' and so on had not yet been simplified.

Of course, France is the only Francophone country whose language rules fell under the jurisdiction of the Académie Française (which now performs a purely advisory role). In some French-speaking regions and countries, they don’t use any base-20 numbers. The Swiss, for example, opt for 'septante', 'huitante' (or sometimes 'octante') and 'nonante', while in Belgium 'quatre-vingt' remains the word for eighty, while 'septante' and 'nonante' are the norm for seventy and ninety respectively.

For more on counting up to 100 in French, be sure to check out Lesson 03 of the Complete French Course.