Do you have to take the second leg of a domestic flight?

Be aware that doing this can go wrong very quickly in the event of "irregular operations", such as bad weather, canceled flights, etc.

When you book a ticket FLL->ORD->STL the airline is committing to fly you from FLL to STL. They are not required to get you there via ORD.

If the FLL-ORD flight is canceled for some reason then it's possible they would re-route you on an alternate flight via a different destination - such as a direct FLL->STL flight, or via a different city, such as FLL-IAD-STL.

If this happens you're going to end up stuck in either FLL or STL. You could try and ask them to route you via ORD, but your success would depend on there actually being other flights available FLL-ORD with seats available.

Well it's not like they will track you down and force you to go to STL however...

once you skip a leg of a itinerary they will cancel the rest of the booked trip including the return ticket.

By booking a ticket you are actually signing a type of contract with the airline and the airlines specifically put in "tariff and fair clauses" that say if you ditch they have the right to cancel the rest of the ticket, fine you and even ban you from the airline. Though the only times they would ban or fine would be for repeat offenders.

They also usually have rules preventing you from buying a new one way return ticket if it matches the same route back, that way forcing you to pay their re-booking fees if you want to change your return date/time.

The thing is to check out the airlines rules and be careful.

If it's a one-way flight, it should be fine. If it's round-trip, though, you may forfeit the return portion of your trip if you don't use all of the legs of your outgoing trip. I suggest checking with the airline before doing this, unless you're willing to forfeit the rest of your itinerary.

My first thought was that you'll have trouble getting to your bags, but as you point out, you won't have any.

In general, there's nothing stopping you getting off. Nobody is forcing you to fly. You may get some raised eyebrows, but you should be able to.

Most airlines will still charge you for that second leg, even if you try to cancel it. But if, as I'm assuming you're doing this, you're getting a cheaper flight this way, then cool, go for it!

International flights on the other hand could cause more of a problem, as you would need a visa for the country you're stopping in. But for example, BKK -> SYD -> AKL (Bangkok to Sydney to Auckland), if you decided to get off in Sydney, you'd need a visa, and if you had one, you may still get some questions from the customs officer :)

You should probably read the rules of travel. Rule 6 of UA contract for carriage states this is prohibited. Below is a clip of the rule. You should read all of rule 5 and 6 for more information.

Prohibited Practices:

1. Fares apply for travel only between the points for which they are published. Tickets may not be purchased and used at fare(s) from an initial departure point on the Ticket which is before the Passenger’s actual point of origin of travel, or to a more distant point(s) than the Passenger’s actual destination being traveled even when the purchase and use of such Tickets would produce a lower fare. This practice is known as “Hidden Cities Ticketing” or “Point Beyond Ticketing” and is prohibited by UA.

If you have no baggage, you are okay with losing the whole value of your ticket if you do so then by all the means go for it.

However a very few airlines will let you keep the rest of the trip even if you made a no-show in on of the segments. The airlines that have such flexibility and I'm 100% positive about them are WestJet and AirTransat.

My daughter had a multi leg flight (around the world) valid over a year.
It included say ...-Dublin-Munich- ... .
She flew Dublin to Amsterdam in another airline and advised the main trip airline (Lufthansa) of this prior to the Dublin-Munich leg - they saved on the deal as they had a paid but unused seat. BUT they consequently cancelled the whole of the rest of the ticket. All unused legs.
Notionally the fine print allows them to do so but they lost absolutely nothing on the deal and made a net gain. Regardless of what they are allowed to do, their behaviour is ethically unacceptable, rules of carriage or other documentation notwithstanding.

Needless to say* my daughter managed to get the other legs reinstated BUT that is what airlines can do, or try to do. The extra flight was not related to them and they were notified in advance and they saved money.

And, no, our family will not be flying Lufthansa every again in this lifetime if at all possible.

• Dr McMahon is very sweet, very polite (usually), very very capable and quite up to taking on a mere airline.

This has just happened to my daughter. Her initial destination was RT Denver/Milan, but she really needed to be in London on Sunday, so she did not take her last leg of the trip (Paris/Milan). Instead, through the same online discount airfare company, she purchased a Paris to London ticket that got her there on time. She could have purchased a Milan/London RT but it was more expensive and took more time and she would have arrived too late in London. Air France cancelled her return flight without any notice, and she was forced to purchase a $1000 one way ticket Milan/Denver, as well has have a lost day at the airport. Apparently AirFrance had sold her ticket to someone else. So...AirFrance was paid twice for the same seat and then sold another seat to the same passenger so she could get home. We are very novice air travelers, and never flew internationally before. However, this is not fair business practice, on the side of the airline. The least they could of done is had her pay a fee and given her a seat on the next flight home.