FlyerTalk Evangelist
Join Date: Nov 2000
Location: Nashville -Past DL Plat, FO, WN-CP, various hotel programs
Programs: DL-MM, AA, SW w/companion,HiltonDiamond, Hyatt PLat, IHF Plat, Miles and Points Seeker
Posts: 11,073
Only restriction is there - there must be at least one seat available for sale.
On one trip last year, I realized I had not booked the companion yet (that's me). (lots of changes, so I often waited, but booked the paid flight early for price).
Day before the trip I realized I did not have a seat. Went to book it and flight was sold out.
For the next 10-12 hours I was checking multiple times an hour waiting for a seat to open up. It finally did and I grabbed it. Based on some threads I found here, others in the same boat have always been able to find a last minute seat.