There is much on the web about "rule 240" which purportedly states that airlines are required by this rule to place you on another of their own or other carrier's flights if you flight is cancelled.
One thing that seems clear from the conflicting information about Airline Rule 240 out there is that this is probably no longer a universal rule followed by all US airlines, and that, like everything else in air travel, whether the airline puts you on one of it's own or someone else's next flight is pretty much at their descretion. From my personal experience however, at least one US carrier did place us on another carrier's flight when our flight was delayed for a prolonged and then unspecified amount of time. On July 26, 2008 Airfarewatchdog posted an article about Rule 240 including a table with the policies of each of the different airlines and links to their Rules of Carriage so that you can get the facts directly from the proverbial horse's mouth. -Gus
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