Prior to 1978, in the event of a delay or flight cancellation, all U.S. airlines were required to offer transportation on a competitor's next flight out if that flight would get the passenger to their destination sooner.
Airlines were even required to put economy class passengers in first class if only first class was available.
I actually took advantage of Rule 240 back in the 1990's when traveling to New York on American Airlines via San Juan. I missed my connecting flight due to a mechanical delay and would have been required to overnight in San Juan had I not asked to be "Rule 240'd" on Continental to Newark on a flight that had just one seat left—in first class, as it turned out. After a bit of back and forth with a supervisor (the original agent I spoke to said "sorry we can't put you in firs class") I was on my way.
The rule was mandated by the now-defunct Civil Aeronautics Board and was incorporated in all airlines' contracts of carriage. The only exception was for "force majeure" (i.e. "Act of God") events, which each airline is free to define as it pleases.
Most airlines have eliminated Rule 240 from their contracts because they're no longer required to have one. But surprisingly, three carriers still have one. Maybe they just haven't gotten around to striking it out.
Those airlines are: Alaska, Frontier, and (surprise!) even United.
Alaska still has language about putting passengers in a higher class of service than what they paid for if that's all that's available. Frontier and United don't.
Of course, all this assumes that there is a seat on another airline that will get you where you're going faster than your cancelled or delayed flight, and with planes so full these days that's not a certainty.
Alaska's contract is here (scroll down to Rule 204AS)
Frontier's is here (scroll down to page 31)
And United's is here (scroll down to Rule 24, page 33, especially subparagraph E). Note United's extensive definition of "force majeure" events, including a "shortage of labor" (which presumably includes crew not showing up for your flight because their inbound flight was late).
And to see how other airlines define "passenger rights" consult our Guide to Air-Passenger Rights
And keep in mind that even if an airline has rid itself of Rule 240, it doesn't hurt to ask. Airlines, when it suits them, routinely put passengers on competitors if there's space available. It's up to the discretion of employees, and you should always ask as humbly and sweetly as humanly possible.
George Hobica is a syndicated travel journalist and founder of the low-airfare listing site Airfarewatchdog.com.