On most modern airlines, flying standby occurs when a passenger travels on a flight without a prior reservation for that specific flight. There are two instances in which passengers usually fly standby. First, a missed flight may require a passenger to fly standby on the next flight to the same destination, as they now lack a reservation. Secondly, a passenger may arrive at the airport early and wish to take an earlier flight listed for that day. They will then attempt to travel standby on the earlier flight, and failing that, proceed to take their booked flight. Standby is never guaranteed. A few standby seats are free, but most will cost you. Flights like Air Canada and Alaska Airlines offer same-day airport standby option that allows customers flying on certain fares to standby for an earlier flight on their day of departure.