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The Metro and RER are two separate sets of train tracks; however, they share many of the same stations for easy transfers. The Metro is within Paris and the RER goes to outlying areas, including the airports, Disneyland and Versailles.  Be aware that Metro Stations and RER Stations typically do not have public bathrooms (toilets). Never turn down an opportunity to use a toilet in Paris, as they are difficult to find and are for "paying customers only" (in fact, like in the Louvre, you have to pay specifically for the toilets to use them). After you determine which Metro or RER Line to take, then to determine the direction of travel (i.e., the correct platform) you need to know the terminus or endpoint of the line. For example, Metro Line 8 terminuses are Ballard and Creteil Pointe de Lac. Even if you figured out which Metro Line to take, if you do not know your end-points of travel, you could end up going in the wrong direction. The RER is the same situation: in this case (especially with the Red A line) the RER line forks, and certain trains are set to go to specific destinations. It is good to pay attention to each stop while you are on the subways. Do they offer a shuttle service? They probably do not. Take the RER as the most economical choice (and the quickest) at 10 Euros per person. To get into Paris, take the RER Blue Line B to Chatelet/Les Halles station.  Air France also operates a couple of buses out of the airport and into the city center. Each way costs 15 Euros a person. They take about an hour to get there, and leave every 30 minutes. You could take a taxi, but it's a pretty expensive option. It'll be at least 50 Euros and there's also a luggage fee. You'll quickly find that the only way to really get around is the Metro. Zone 1 encompasses enough of main Paris where nearly all of the sights and attractions are, that it is usually not necessary to purchase the pass to include zones 2-6. Purchase a map if you'd like and you'll be able to see all the routes sprawled out in front of you.  Use the Navigo Découverte pass beyond the cost-savings mentioned above to not have to fuss with paper tickets. While purchasing a carnet (a ten pack) still saves a ton of money and hassle (as opposed to purchasing one Metro pass at a time – which is ridiculous), still, a carnet is nothing more than ten loose tickets – not even held together with perforation. It's easy to mix up used and unused tickets. Hold on to the ticket to prove that you have valid admission. That being said, rarely does the train conductor ever check. Paris is divided into 20 arrondissements, or neighborhoods, which are like distinct sections of town. The first arrondissement is in the center of Paris, and each subsequent arrondissement spirals outward like a seashell.  Paris is divided into two parts by the Seine River (la rive gauche (left) and la rive droite (right)). On the West side (la rive gauche) are the Arc de Triomphe and the Eiffel Tower. In the center is the Louvre and the Musee d'Orsay. Slightly East (though also central) is Notre Dame. The university and student areas for young people are also on the East side. Just north of center is the Opera shopping area, where a single name brand product might be represented by a single store that takes an entire city block.
Know the difference between the Metro and the RER. Figure out how you're getting from the airport to the hotel/apartment. Use the Metro. Get a feel for the geography and layout.