This article is from the Paris, France FAQ, by Thierry E. Toniutti and Delphine Kensit nemo_n@yahoo.com.
The basic RATP ticket is a 2 zone ticket, valid for the entire metro
system, as well as the relevant zones for the bus and the RER. RATP
tickets are valid indefinitely, so the tickets you have left over from
your trip 3 years ago are still valid today! Do remember to keep your
ticket with you at all times. Ticket inspectors are a common sight in
the metro and you will be fined 350fr (or 100fr if you pay on the
spot) - no excuses entertained. In addition, you will need your ticket
to exit the RER system - so keep it handy. All transfers within and
between the metro and RER are free. Transfers to and from the bus
require a new ticket.
Metro tickets can be purchased singly at 8FRF each, or in a set of ten
(un carnet de dix, or simply, un carnet) for 55FRF. Tickets can be
purchased at every metro station from the automated machines, or from
the ticket agent. These can also be purchased from Tabacs displaying
the green RATP sign. The RATP does not recommend that you buy tickets
from unlicensed vendors. You can typically see them loitering near
automated machines at the major train stations.
There are special priced tickets available for children 10 years and
under. These are referred to as "tarif réduit" and cost half the price
of an adult ticket.
If you need to travel outside of the 2 zones, you will likely need to
purchase your ticket from the ticketing agent. Some stations have
automated machines that allow you to purchase tickets for more than 2
zones, but they are not very common. Specify the station that you need
to go to, and consider buying your return trip ticket also. This can
save you a lot of time and headache if you are travelling to
Versailles on a weekend.
 
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