[QUOTE=House]Generally speaking an airline like UA is likely to have current interline agreements with nearly every other major airline. \[QUOTE]

Well, that's what I would have thought too, but I discovered just last week that UA does not interline with Aerolinas Argentinas (AR), the flag carrier of one of the world's major nations, and a destination UA serves twice a day from IAD and ORD! So I guess you really do have to check.