RER/Metro ticket

You are correct. You must hold on to the ticket and use it to enter the metro for that part of your trip.

You don't use it to switch to the metro since you're still inside the system. You use it to exit the system when you head for daylight at the final destination.

My recollection is you do both, but I have not been to Denfert-Rochereau station. Use the ticket to exit the gate at the RER part of D-F station and then use it to get trough the gate in the Metro portion of the station. IF it did not work that way, you could board the RER at CDG or any of the outer stations with a cheap single zone ticket, then exit to the street for free. That is how I recall it working at Les Halles, but D-F could have a different configuration. Metro stations do not have ticket enabled exit gates since the Metro system is all one price. The gate opens as you approach it from inside the system.

The last time that I took the RER and changed to the metro, it was as Sam described. You need to hold on to the ticket until the end of the trip -- first to make the transfer from the RER to the metro and then until the end of the trip on the metro in case you are stopped by a ticket inspector.

You guys are right, if there's a turnstile twixt the two systems. Recycle the ticket.

Thanks, guys!

The RER is metered for distance traveled and requires specific point to point tickets for places outside Paris so you use the tickets to both enter and exit -- the metro is one size fits all and so you only need the ticket for entry (and if inspected along the way -- keep the ticket or risk a hefty on the spot fine) Coming from CDG which is well outside Paris, you use the ticket (9.75 CDG-Paris) to enter the stile at CDG and then to exit at DEnfert Rochereau -- you then re-insert the same ticket into the entry stile for the metro which will 'know' it is an ongoing ticket. (sometimes this step is not needed -- I know it is at Gare du Nord and seem to recall it is also true at Denfert) Throw the thing away only after you exit the station to the outside so it doesn't get mixed up in you unused tickets.