In French as spoken in France and Canada, the numbers 70 and 90 are strangely formed:

  • 70 is soixante-dix (sixty and ten)
  • 80 is quatre-vingts (four times twenty)
  • 90 is quatre-vingt-dix (four times twenty and ten)

Seventies

More logical words are used in Belgium and/or Switzerland:

  • 70 is septante (from 7, sept)
  • 80 is octante (from the latin prefix for 8, octo)
  • 90 is nonante (from 9, neuf)

The reason for these strange words is historical. Whereas the Romans were using a decimal system, the Celts were using a vigesimal numeral system, based on multiples of 20 and not 10.

Somehow, after the conquest of Gaule by the Romans, the decimal system won from 0 to 60 but the vigesimal system remained from 60 to 100, which explains the current schizophrenic system!

Interestingly, other civilizations have used vigesimal systems, for example Mayas and Aztecs in South America and Ainus in Asia.

Twenty 

Finally, we all know that the decimal system has been chosen because we have 10 fingers, so maybe our ancestors were also using their toes to count!