this this routing 'legal'? (2 x 1 way vs round trip)
 

Hey folks - I usually dont have to get creative with routing, and I suspect this is fine, but wanted to ask before I booked it.

I've got to do SFO-DEN soon and then DEN-LAX-SFO.

The first part, SFO-DEN, is work, the second part is on me. I'd like to book them as two separate one way itineraries. But since I'm starting and ending at SFO, will that be a problem?

Not a problem at all booking two one way tickets instead of one return.

much appreciated - thanks SamOF!

If you book the two separately, there should be no problem. And within North America - Caribbean, there’s generally no one way penalty pricing. You might want to try pricing these using ITA or Google Flights to compare for pricing.

By "legal", I assume you are asking if it violates the ticketing rules in the contract of carriage. No, there's no problem there. There are just a couple cases where there are potential CoC violations. Once case is if you nest multiple tickets to circumvent minimum stay requirements with roundtrip fares (which are you are not doing). The other cases involve hidden-city and throwaway ticketing where you buy a ticket, but do not intend to fly all the segments. For example if you bought a DEN-LAX-SFO fare because it was cheaper than a DEN-LAX fare and intend to just skip the LAX-SFO segment. Throwaway ticketing is when you buy a roundtrip ticket because it is cheaper than a one-way fare, and then just use it as a one-way (this is fairly rare for domestic fares these days, but it's still quite common for international roundtrip fares to be cheaper than one-way's).

Interesting!
So, there is a slight, like 10% chance that I have to do a day trip in the middle of this trip... I might have to do EGE-ORD-EGE ... If I book three separate trips:
1. SFO-DEN
2. ege-ord-ege
3. den-lax-sfo
is that still legal?

Sure. Not an issue for one way tickets as they have no length of stay requirements like round trip tickets do
Nested round trips might violate a rule, flying like
1. SFO-DEN Out Monday (roundtrip fare with #3 )
2. DEN-SFO-DEN Out Friday, return Monday would be an issue if the roundtrip fare for #1 and #3 has a Saturday night stay requirement
3. DEN-SFO returning the next Friday as part of a roundtrip fare with #1
This was a bigger problem in the old days, as now you can usually get one way fares that are as low as 1/2 of a roundtrip fare, at least domestically

Yes, that's fine. There's no attempt to circumvent a roundtrip minimum stay requirement and you intend to fly all the segments.

Depends on the. Where you have competitive presence, one way is often the same as round trip. Where the market is limited in size or coverage, airlines often extract a premium. I saw that first hand last week.

Basically there's no rule that I can think of that would ever make any series of one way tickets illegal.

All of the ticketing rules you're thinking of have to do with not letting you circumvent minimum stay requirements etc on roundtrip fares in situations where the roundtrip fare is cheaper than two one ways.

Yep. Based in DTW and there are numerous domestic markets where DL still has roundtrip fare filings (that are cheaper than two one-way's). For example, the cheapest DTW-DSM one-way fare is $202, while they have roundtrip fare filings for as low as $227 roundtrip.

Combining both market dominance and round-trip necessity: DCA-GSO was $684 round trip and $849 if you booked two one-ways. Such a deal for a 45 minute flight....

Yes, my understanding is that's classic back-to-back ticketing, which is generally prohibited by an airline's contract of carriage. The two workarounds that should be totally legit are (1) buy SFO-DEN leg #1 and DEN-SFO leg #4 as (more expensive) one-way fares., or (2) fly the nested DEN-SFO-DEN on a different airline, not only will it make it impossible for the airlines to know you're doing it, they really can't do anything to prohibit it.

But nesting a trip to another city inside another round-trip is fine. Of course I guess workaround #3 to the above (if Sat night stays were an issue) is to nest, say, an ORD-PHL-ORD inside an EWR-ORD-EWR if you lived in central Jersey.

These are separate tickets. The risk is all on OP. If he has to cancel or make changes to his trip, each separate ticket will incur the $200 change fee, so a very expensive cancellation if it comes to that (and he's on inflexible tickets).

This is super helpful, thanks everyone for chiming in!

One more hypothetical - could I do this:

Ticket 1:
Monday: SFO-DEN .... Following Monday DEN-LAX, Tuesday LAX SFO.

Ticket 2:
Saturday: EGE-ORD-EGE

In other words, can I buy a separate ticket for that one day turnaround to Chicago and back in the middle of another round trip ticket? I think I've always thought that wasn't allowed...

(of course, the other thing is that it looks like AA no longer has a Saturday EGE-ORD flight, so it's probably a moot point)