I address this to anybody, in general, but I'm assuming that Fraggle will have the most accurate answer. Why is it that "Seventy" in French is "Sixty-Ten"; "Seventy One" is "Sixty-and-Eleven" and so on and so on? While digging for the answer, I discovered that some Francophones actually do have an independent word for the "Seventies": Septante. Similar phenomena for "Eighty" ("Four-Twenty") and "Ninety" ("Four-Twenty-Ten")! So, what gives? ~String
I don't know about origins, but in Belgium we have "septante" for seventy and "nonante" for ninety. I've heard the French-speaking Swiss also have an "octante" for eighty. This reminds me that the human lifespan is supposed to extend over "three score and ten" years.
Here's an explanation from the Académie Française, and here's an English translation: I have seen a conjecture that in addition to the substratum of a vigesimal counting system carried over into the Gallic dialect of Latin by the Celtic Gauls and Germanic Franks, there was an (earlier? later?) substratum of a duodecimal counting system, i.e. by twelves. Vestiges of a duodecimal system are rampant in European culture, e.g., our day broken into two groups of twelve hours, our year subdivided into twelve months, our eggs and bakery goods sold in groups of twelve. Sixty is a special number to people who use both the decimal and duodecimal system, because it is the smallest non-zero integer that is a multiple of both ten and twelve. We divide our hours into sixty minutes and our minutes into sixty seconds; the number of degrees in a circle is a multiple of sixty. Perhaps the medieval French felt the pull of that substratum and kept their old way of counting up to sixty, their special number. And then when they got to eighty and ninety, they just kept going from sheer inertia. But when they got to one hundred, in a numbering system that, despite their cultural history, was unequivocally decimal, I can easily see them succumbing to common sense, reverting to standard Latin and calling one hundred cent, instead of encumbering themselves with cinq-vingts and cinq-vingts dix. (I'm not trying to capture the medieval spelling, "Modern" French spelling is difficult enough.) The Belgians and Swiss, on the other hand, were members of multi-ethnic communities and were influenced by their Dutch- and German-speaking neighbors, respectively, both of whose number systems are uncompromisingly decimal.
