I've read about using a carnet, but I don't know what they look like. It sounds like a packet of 10 (?) tickets sold at a discount. My question is whether two people can share those 10 tickets or whether each person has to purchase his own? Thanks for your help.
The carnet is just a stack of tickets, folded on the perforations. Any number of people can use the tickets - it's just a way to buy 10 tickets cheaper than one and a time. Since it's best to see Paris above ground, 1 carnet is plenty for a few days. We walked all over, then would take the metro back to save our feet.
When you get to a metro station and go downstairs, you will see a ticket window. You can buy a carnet of tickets there. Tell the clerk you would like one carnet, and a printer shoots out a stack of ten small tickets. The tickets are good on the RER also. If you use one to ride the RER, remember to hang on to it--you will need to insert it into the turnstile in order to exit the station. You do not need to do this to exit the metro. In either case, rip the ticket in half so you do not mistake it for an unused ticket. When we visited, we used almost one carnet per day, but it is not something you need to decide ahead of time. Buy one carnet, and when you are running low, buy another. The metro is so convenient and efficient!
One clarification: the metro tickets are good on the RER lines which run in the city. If you are taking the RER outside of the city, say, to Versailles, you cannot use the metro tickets.
Here are a few more details. While a single ticket is good anywhere the Metro goes (including the few lines which end in zone 3), it's limited to zone 1 on the RER. So long as you don't exit the system, you can make as many transfers as you wish. However, each bus ride requires a separate ticket. There are several kinds of passes. The best one, if any, for you depends upon how long you'll be in Paris, which days of the week you'll arrive and depart, and whether you arrive by train or plane. If you post that information, I'll be happy to give you a pass breakdown.
The reason why I thought a carnet would work best is because I'm arriving on a Thursday and leaving the following Tuesday. Also, I don't plan on taking any trips outside the Paris city limits. I've already seen Versailles. The last time I was in Paris I didn't use the metro at all. This time I definitely want to use it. Thanks for everyone's help. I'll be staying near Madeleine Plaza very close to the Madeleine metro stop, so I want to take advantage of it.
With your schedule, a Carte Orange is not possible. The Paris Visite pass is overpriced. If on any given day you know you will take more than five Metro, RER, or bus rides, then a one-day Mobilis pass for zones 1-2 would be a good choice for that particular day. The cost is 5.50 EUR. Otherwise, just buy carnets as needed.
Angel: You'll notice that understanding the metro system is easy as pie. The added bonus? No matter where you are in Paris, you can be very confident there's going to be a subway stop in walking distance.
Don't overlook riding the buses to get around. The Metro is fast, but you can't see much of the city while riding it. Every bus stop/shelter has a large detailed map on the back showing where each route goes, and the stops are shown in straight-line representation inside the bus just like a Metro carriage. As mentioned above, however, no transfers; each bus ride takes another ticket.