Tip: The RER , while it looks similar to the Metro, is actually a different train system that mainly serves the areas outside the city. So when you're traveling throughout the city make sure you're using the Metro and not the RER. Metro stations will be labeled with a large "M". Always note the zone to which you want to travel to be sure that you buy the appropriately priced ticket. If you are caught on the metro either without a ticket or with a ticket that does not cover you for your trip, you risk being fined if there is a control. Example: While your metro ticket can take you out to La Defense on line 1, should you use the RER A to get there, you need to have a ticket priced for zone 3. You need to re-introduce this ticket into the turnstyle in order to be allowed to exit. Hang on to your tickets for the whole ride.