If you’ve ever embarked on the noble journey of learning French, you’ve probably hit “The Wall.” It’s not a grammatical concept or a tricky verb conjugation. It’s a number. Or, more accurately, a series of numbers that suddenly abandon all logic you thought you’d mastered. Everything is going swimmingly—un, deux, trois, dix, vingt, trente, quarante, cinquante, soixante… and then, chaos.

Instead of a simple, unique word for seventy, you get soixante-dix (sixty-ten). Okay, a little weird, but it’s just addition. You can handle that. But then comes eighty: quatre-vingts. Four-twenties. You pause. You check your notes. Did your teacher just ask you to do multiplication to say a number? Yes. Yes, they did. And it continues with quatre-vingt-dix (four-twenty-ten) for ninety.

This baffling system, a stark contrast to the straightforward decimal counting in neighbors like Spain (ochenta) and Italy (ottanta), is not a mistake or a modern French prank on language learners. It’s a living fossil—a fascinating linguistic relic of a partial vigesimal, or base-20, counting system that whispers tales of ancient Celts, raiding Vikings, and the very way our ancestors perceived the world.

The Decimal Comfort Zone and France’s Departure

Most of the world runs on a decimal, or base-10, system. It’s intuitive and feels completely natural. We have ten unique digits (0-9), and once we hit ten, we start combining them. The logic is widely believed to stem from the most convenient counting tools we possess: our ten fingers. It’s simple, efficient, and globally dominant.

Romance languages like Spanish, Italian, and Portuguese inherited their decimal systems directly from Latin, which was strictly base-10. So, their numbers progress in a beautifully predictable pattern:

• Spanish: sesenta (60), setenta (70), ochenta (80), noventa (90)
• Italian: sessanta (60), settanta (70), ottanta (80), novanta (90)

French, a Romance language itself, followed this pattern… up to a point. At sixty, it takes a sharp detour. Let’s break down the math:

• 70 = soixante-dix (60 + 10)
• 80 = quatre-vingts (4 x 20)
• 90 = quatre-vingt-dix (4 x 20 + 10)

This isn’t a full base-20 system, where every number is conceptualized in twenties, but a hybrid. It uses base-10 until 60 and then weaves in base-20 logic for the remaining numbers before 100. This mashup points to a powerful historical influence that was strong enough to overwrite the “standard” Latin system in this specific numerical range.

A Linguistic Detective Story: Who’s to Blame?

The question of where this vigesimal quirk comes from is a historical whodunnit with two main suspects: the Celts and the Vikings. Both cultures used base-20 counting and both had a profound impact on the land we now call France.

Exhibit A: The Celtic Hypothesis

Long before the Romans arrived, France was part of Gaul, a vast territory dominated by Celtic tribes. The Celts had a well-established vigesimal counting system. The logic is simple: if ten fingers give you base-10, what do you get when you add ten toes? Base-20. It’s a “full body” counting system.

Evidence of this system survives in modern Celtic languages, providing a strong parallel to French:

• In Welsh, 20 is ugain. 40 is deugain (two twenties), 60 is trigain (three twenties), and 80 is pedwar ugain (four twenties)—a near-perfect echo of quatre-vingts.
• In Irish Gaelic, 20 is fiche. 40 is dhá fhichead (two twenties), and 60 is trí fhichead (three twenties).

According to this theory, the vigesimal system was the common way of counting in Gaul. When the Romans introduced Latin, the base-10 system took over for most applications, but the old base-20 habit clung on for everyday numbers, eventually fossilizing into the French language.

Exhibit B: The Norse Hypothesis

The second suspect arrived much later. Starting in the 9th century, Vikings, or Norsemen, began raiding and eventually settling in northern France. Their territory became known as Normandy (“land of the Northmen”), and their language, Old Norse, also featured a vigesimal counting system.

The Normans were incredibly influential. After conquering England in 1066, they infused the English language with a massive amount of French vocabulary. It’s plausible that their high status and influence within France helped preserve or even re-popularize the base-20 system, which may have already been in use from the Celtic era. The English word “score”, meaning a set of twenty (as in “four score and seven years ago”), is a remnant of this same counting tradition, likely from Norse influence.

Linguists today often believe it wasn’t an either/or scenario. The most likely explanation is that a pre-existing Celtic vigesimal system was later reinforced by the influential Norse-speaking Normans, cementing its place in the evolving French language while Latin-based decimal counting dominated elsewhere.

The Rebellion: Not All French Speakers Count This Way

Perhaps the most compelling evidence that quatre-vingts is an anomaly is that many French speakers don’t even use it. In parts of the French-speaking world, the logical, decimal-based numbers won out.

• In Switzerland, you’ll hear the much simpler septante (70), huitante (80), and nonante (90).
• In Belgium and the Democratic Republic of Congo, they use septante (70) and nonante (90), though they typically stick with quatre-vingts for 80.
• Even within France, some rural eastern regions historically used these simpler forms.

The persistence of the vigesimal system in Standard French is a testament to the power of Paris. As the capital’s dialect became the national standard, its quirky counting system was exported across the country and its colonial empire, overwriting the simpler forms used elsewhere.

Multiplication Before You Speak: A Cognitive Quirk

Beyond the history, there’s the simple cognitive reality of using the system. To say “95”, an English speaker retrieves a single unit: “ninety-five”. A French speaker, however, must perform a mental calculation: “four-twenty-ten-five” (quatre-vingt-quinze). It’s a multi-step process of multiplication and addition that happens in a split second.

Does this make French speakers better at mental math? Probably not. Studies on the topic are inconclusive, but most linguists agree it adds a slight, almost unnoticeable, cognitive load. It’s a tiny mental speed bump on the road to communication, one that native speakers navigate effortlessly but that stands out as a mountain for learners.

A Relic in a Digital World

So, the next time you hear or say quatre-vingts, remember that you’re not just stating a quantity. You are using a “fossil word” that carries the echoes of ancient traditions. It’s a testament to the fact that language isn’t always engineered for maximum efficiency. Instead, it’s a beautiful, messy, living museum of human history, packed with stories of conquest, culture, and the different ways we’ve found to make sense of the world—even if it means doing a little multiplication just to say “eighty”.