I just checked http://www.sncf.fr the French rail website and it says:

On other SNCF rail services, reservation guarantees your seat's availability for 1.5 # and entitles you to reserve seating in a smoking/non-smoking coach, in a compartment or a coach with central aisle subject to seating availability.

When to reserve: Seat reservations are accepted#

* on TGV lines, between 60 days and a few minutes ahead of travel. * on other mainline services and regional rail networks (where available), from 60 days before departure: * up to midday for a same-day departure after 5 pm; * up to 8 pm for a next-day departure before 5 pm.

So go to: http://www.sncf.com/indexe.htm Choose Information and Booking Choose Passenger's Guide and then Reserve your seat.

By the way, we found out that we needed to stamp our TGV reservation cards at the machine on the platform before boarding the train. We thought that our rail pass was our ticket (no stamping of this, just the date put in by us). If you don't have a rail pass, you don't have a separate reservation card, I believe.

Frances Toronto, Canada