Sorry if this has been asked before, but let's say that airline XYZ offers a daily transcon SEA-ORD-MIA. Some passengers are booked and assigned seats only on the SEA-ORD leg. Others are booked and assigned seats only on the ORD-MIA leg. Still others are booked and assigned seats for the entire journey.

What happens if a last minute flyer wants to book a ticket and receive an assigned seat SEA-ORD-MIA, but there is no one seat that is still open on both legs. Each segment has available seats, but on the two-segment flight every seat is occupied for at least part of the trip. Ex: (9A is vacant on the first leg, but is occupied on the second leg.)

How would the airlines handle this? Would they issue the last minute flyer 2 separate boarding passes for each leg and make him change seats at the stopover in ORD? Would they change someone else's seat assignment who was only traveling on one segment so that the last minute flyer could be assigned the same seat on both legs? Would their reservation system show no more available seats on the 2 segemnt trip (even though both segments have vacant seats on each leg - just not the same ones)?

How do the airlines balance this out? I'm also wondering if this is another reason that Southwest doesn't have assigned seating, as a lot of their long-haul service involves the same plane, but with multiple stops.

Mike