as a rule of thumb. Even if its a partner flight that AS allows you to select seats on, ALWAYS go double check on the partner airlines site. That doesnt just go for AS partner flights. That goes for any mixed airline itinerary. Ive had many times where I chose stuff on AS or another airline to have it show no seat selected on the other airline or it auto selected me a seat. Part of it is because not all airlines have the same seat numbers and letters. AA for instance doesnt have a seatC or D in the first class cabin on 737s. Like wise on AS F there is no B or F. AA also has a row 5 on 737s, AS doesnt. That was a while back too. Now AA just renumbered rows. DL doesnt have rows 6 through 8 on most 738s