If you're a frequent flier in the friendly skies, you have probably heard about that mysterious "Rule 240" invoked by angry travelers whose flight — maybe the last one of the night — was canceled. What is this rule that is wielded like a club by disgruntled air passengers?

In its day, Rule 240 was perhaps the most pro-passenger regulation ever enacted to protect air travelers. It addresses the obligations that an airline has to passengers when their flights are canceled or delayed, or connecting flights are missed because of the actions or omissions of the carrier. What it does not cover are flight delays and cancellations that result from natural causes like inclement weather or from other factors that are beyond an airline's control.

Just about everybody who flies knows — painfully — that the airlines were deregulated in 1978. Prior to that year, Rule 240 was a U.S. federal requirement under the old Civil Aeronautics Board, one of the many stipulations by which the U.S. government obligated airlines regarding the transport of passengers and the rights of customers. The concept of Rule 240 has remained in some form or another since 1978's deregulation free-for-all because domestic airlines continue to respect the old rules of the skies. But it is often buried in the airlines' contracts of carriage that most passengers never read. Big mistake. Every major U.S. legacy carrier has incorporated some form of the rule into its own contract, or conditions, of carriage that binds every ticket purchaser into a contractual agreement with the airline regarding transport. Airlines often subscribe to different versions of Rule 240, making it even more difficult for passengers to determine their rights without studying the contract. It may appear in disguise among the mind-numbing paragraphs of the contract of carriage. American Airlines, for example, calls it Rule 18; Continental refers to Rule 24; and US Airways masks Rule 240 as Section X.

Passengers cannot invoke Rule 240 unless the airline is at fault for cancellations or delays. Reasons would include mechanical problems, late arrival of an aircraft, pilots and crew late for duty, and other factors over which a carrier can exercise control.

If Rule 240 kicks in — and keep in mind that it is now a voluntary airline policy, not a federal regulation — the carrier, upon request, might offer confirmation either on its next flight or on another airline, hotel accommodation, meal vouchers, phone calls, hotel transfers, and maybe even the option to obtain a refund if the traveler does not wish to proceed with an air itinerary.

Because some airline airport agents are vague on the existence or wording of a carrier's Rule 240, savvy travelers don't leave home without a copy of the specific policy. You need to know in advance what you can expect in case of flight cancellations or delays.

Some industry-watching experts warn against actually mentioning the term Rule 240 to an agent, as doing so could make a passenger appear to be a complainer that no agent likes to go out of his way to please. Instead, the experts suggest that you refer the agent to the terms of the contract of carriage or conditions of carriage that you have cleverly brought for reference. Or politely ask the agent to "240 me" on an alternate flight when you see "Canceled" flash on the arrivals/departures board. If you're going to negotiate for perks or compensation, bite your tongue and be calm and super-civil.

Prudent folks make a point of reading not only the Rule 240 clause but also the entire contract between a passenger and the airline. It includes a number of passenger rights, some federally required, that you probably aren't aware of — everything from issues of refund entitlement to overbooking compensation. These contracts, by law, must be made available to you by the carrier upon request, and most are downloadable from the airline's Web site.

Contracts of carriage are not graven in stone; they can and do change frequently. United Airlines, for example, has changed its language to say that in the event of a delay or cancellation, it will still fly you on a competitor, but not necessarily in the same class of service as on your original United flight. Delta's Rule 240 now omits the section in which it used to say it would fly you on another carrier in the event of a "flight irregularity." American's version promises only to place you on one of its own flights. Alaska and Northwest airlines have stayed with most of the original 240 language. Considering the general tendency of airlines to reduce passenger amenities and services, Rule 240 and its doppelgangers by any other name could eventually disappear into the ether.

Gate and counter agents still have the discretion — and usually the power — to act in the spirit of the old Rule 240 and help passengers when a flight is delayed or canceled by anything other than weather. For the past 30 years, you haven't been able to demand action under Rule 240 because it's not a federal regulation. But you certainly can ask. And people do this successfully every day. Which brings me to an important rule of life: If you don't ask, you don't get.

Irene Croft Jr. of Kailua, Kona, is a travel writer and 40-year veteran globetrotter. Her column is published in this section every other week.