What is the authoritative source for how to write French numbers?

Question

I am not sure how to write French numbers because teaching websites do not all follow the same rules, in terms of the hyphens, use of "et," etc. Here is an example of two sites that show up when I google French numbers from 1 to 100:

https://www.fluentin3months.com/french-numbers and https://www.frenchlearner.com/numbers

And some differences between what these two sites teach:

• Soixante et un vs. Soixante-et-un
• Quatre-vingt-et-un vs. Quatre-vingt-un

I appreciate a link to a reliable site so I can learn the numbers properly. Bonus if also includes examples of using thousand, million, billion, etc. Thanks.

Answer 1

Both soixante et un and soixante-et-un are correct, the former uses the traditional spelling while the second follows the 1990 rectifications.

Quatre-vingt-et-un is never used to express 81 (Quatre-vingt-un). The word itself exists but is the name of a dice game (4-21), more commonly called quatre-cent vingt-et-un (421).

Wikipedia has a decent page explaining the French numbers rules.