21,31,41,51,61,71 - tous avec un trait d'union?

Question

I've noticed that 71 above has no hyphens: soixante et onze. So is it just the numbers 21,31,41,51,61 that do have hyphens? Or does 71 have hyphens just when there are higher numbers, e.g. 171?

Merci d'avance!

Accepted Answer

In 1990 the Académie Française reformed the spelling of numbers below 100: all parts of a multi-part number are connected by hyphens. This, however, is only a recommendation which is why there sometimes is a heated debate over how to spell certain numbers.
I always connect them with hyphens, particularly since this removes ambiguities such as:
soixante et un tiers (60 + one third)soixante-et-un tiers (61 thirds)