[FabHK]

But the etage above the Rez-de-chaussée is 1, correct? The Rez-de-chaussée then is 0. That is 0-based indexing.

  US  D  F
   3  2  2
   2  1  1
 __1__0__RdC
  B1 -1  je ne sais pas

[skydhash]

There is no Etage 0. People will just confusingly look at you if you ever said that. And the "Je ne sais pas' is called "Sous-sol"

[gwhl]

In a lot of elevators rez-de-chaussée is labelled 0. We just don't use that when communicating

[scraptor]

You can have multiple basement levels, which get numbered again. 1er Sousl-sol, 2eme Sous-sol etc. The ground floor might not be explicitly labeled as zero but the number between "one up" and "one down" is still zero.