Oliver Keating:
The metro has a fare system very similar to LU, with zones (I think there
are about 6). Unlike LU, most of the *metro* is confined to zone 1 (which is
clearly identified on all metro maps). However, the RER forms a much larger
Paris-wide network where the other zones come into play.

Also, the Metro is flat-fare for the whole system. Zone 1 is the whole
city of Paris, but where the Metro does go outside of it, the regular
fare still applies. This means that for places like La Defense (zone 3)
that you can reach from Paris by either Metro or RER, the RER may be
faster but the Metro is cheaper. I remember that some years ago that
particular trip was about 5 francs by Metro, 10 francs by RER second
class, or 15 by RER first class (since abolished).

Within Paris, of course, the same fare applies to both Metro and RER,
and you can transfer between them on a single ticket.
--
Mark Brader, Toronto | Do not meddle in the affairs of undefined behavior,
| for it is subtle and quick to anger.

My text in this article is in the public domain.