I'm learning my French numbers now, and it seems that in order to learn to count in French you must utilize arithmetic — specifically, addition and multiplication. In most of the civilized world, children learn the numbers first, and then learn to use those numbers in mathematics, but in France you need to know some mathematics in order to learn the numbers! This is the Mobius strip of math, a numerical hall of mirrors. To count, you must multiply, to multiply you must count, to count you must multiply…
The fun begins at seventeen, which you construct by adding seven to ten: dix-sept. Eighteen is dix-huit, or ten-eight, nineteen dix-neuf. Twenty is vingt, thirty is trente, and so forth, and you form these two-digit numbers above twenty the same way you do in English, with the root, a hyphen, and the next numeral. Thus twenty-two is vingt-deux;
All is well, in fact, until soixante-neuf. If you don’t speak French and that number rings a bell, congratulations: you’ve read your Kama Sutra (or your Joy of Sex, which, to its credit, used the classy French numeral rather than the cruder English “sixty-nine”). Soixante-neuf is the last “easy” number in French. Should you want to turn your love-making up a notch to seventy, you’ll find out there is no “seventy” in French. Why would you need to waste a word on the decuplet from seventy to seventy-nine when you can simply add ten to sixty, and arrive at…soixante-dix? Likewise for next nine numbers: seventy-one is sixty-plus-eleven, right up through sixty-plus-nineteen. But nineteen, remember, is itself ten plus nine! So seventy-nine is sixty-plus-ten-plus nine: soixante-dix-neuf.
Whew! Thank goodness we’re up to eighty, and can stop adding.
And start multiplying. I kid you not. Eighty is quatre-vingt, literally “four twenties.” Guess what ninety is: quatre-vingt-dix. That is, 4 times 20, plus 10. You continue in this fashion until you hit 99: quatre-vingt-dix-neuf. So from 1 through 60, the French use from a base-10 (decimal) numerical system, after which they switch to a base-20 (vigesimal) system. There are other cultures, including the Mayans and the Basques, who use a vigesimal system, but to my knowledge, French is the only language that uses a mixture of decimal and vigesimal, most likely as a result of unification of several number systems in existence in France after the Revolution. It’s enough to make you fou!