How do airlines account for other airlines miles?
 

I was wondering how airlines account for awarding and redemption of other airline's miles. When you claim miles for a flight from a different program(claiming AA miles on an Alaska Airlines flight), how does the airline account for those miles? Do airlines buy miles from each other, or just swap miles and hope an equal number of passenger will claim their miles on partnership airlines.

[This message has been edited by Tango (edited 03-20-2000).]