In order to fly stand-by you need to be ticketed for another flight the same day. For example: I am ticketed on a 4PM flight from SEA to PHX but my plans change and I can leave earlier so I show up at the airport and I'm placed on the stand-by list for the 1PM flight. If there are open seats on the earlier flight, and if the rules of the fare I purchased on the 4PM flight permit stand-by, I would be allowed to fly on that earlier flight. If the flight is full, no harm done because I still have a confirmed reservation on the later flight. Alaska Airlines permits same-day standby on most of their fares. Other airlines now charge a fee to stand-by and some don't permit it at all forcing you to ay a ticket change fee and usually additional fare if you want to take a different flight on the same day even if the flights are not full. Hope this answers your question.