The Paris Métro and RER suburban train systems are linked at some stations. Often this means you can ride the Métro, exit that system, then buy a ticket from a ticket booth and enter the RER system.

However, at some smaller stations the systems may be linked so that you walk directly from the Métro to the RER, with no ticket booth or ground-level exit available. You may be trapped!

If you have an RER ticket in hand, or are using an electronic transit pass such as Navigo valid for both systems and all the zones to your destination, no problem! You use it to pass the barrier between the Métro and RER systems and you're on your way.

But if you have no RER ticket, you will have to return to the Métro system and ride the train until you find a ticket booth!

By the way, the barriers that separate the Métro and RER systems are often just that: barriers designed to keep you from passing from one system to the other without a valid titre de transport (ticket or pass). They are not just turnstiles which you can sneak over or under, and sometimes there is no agent nearby to help you out of your predicament.

Thus I strongly suggest that if you plan to ride the RER B train from central Paris to Charles-de-Gaulle Airport, you buy your RER ticket the day before you travel. That way, if you happen to encounter one of these direct station links with no ticket booth, you'll have what you need and you won't delay your trip to the airport by having to return to the Métro and ride a train until you can find a ticket booth.

Also remember that you will need your RER ticket to exit the RER system at CDG Airport. If you have no ticket, you will not be able to exit—there are barriers between the RER system and the airport. There may not be an RER agent to help you, and the barriers will prevent you from entering the airport.

(If you find yourself in this predicament, note that it's easier to get help at Aérogare 1 than at Aérogare 2.)

If you plan to use the RER B line to travel from central Paris to CDG Airport, buy your RER ticket the day before you travel.