Reserving a seat on the TGV (Train à Grande Vitesse) in France is mandatory, as all high-speed tickets in 2026 include a specific seat assignment by default. When you purchase a ticket through the SNCF Connect app, the website, or at a station kiosk, your seat is automatically reserved as part of the transaction. You can often choose your preference, such as "window" (fenêtre), "aisle" (couloir), or "traveling in the direction of travel." If you are using a Eurail or Interrail Pass, you must purchase a separate "Seat Reservation" before boarding, which usually costs between €10 and €20 for domestic French routes. These can be booked online via the Eurail website or at an SNCF ticket window. It is crucial to have this reservation printed or on your phone, as conductors on the TGV are strict; boarding without a valid reservation can result in a significant fine, even if the train appears to have empty seats, as the system is designed to prevent overcrowding on the high-speed network.