keithguy can probably provide a clearer explanation than can I, but I'll give it a shot.

Ticket 1: A-B-A
Ticket 2: B-C-B

Any such tickets where Ticket 2 is scheduled between the departure and return of Ticket 1 are defined as nested. But nested tickets, in most cases, are legal.

For a ticket to be illegally nested (that is, back-to-back), three things must be true:

(1) C must be the same airport as A, or a co-terminal thereof. Note that this applies only to the destination airport so far as I'm aware, and not to any connection points in between. For example, I can book SFO-ORD-SFO and then ORD-SFO-SMF-SFO-ORD without running afoul of these rules (since my destination point on Ticket 2 is SMF and not SFO).
(2) The tickets must be booked on the same airline (with the possible exception of code-shares -- this is where I'm a little fuzzy).
(3) The ticket must be booked in such a way as to deliberately be circumventing the airline fare rules (e.g. booking to avoid a Saturday-night stay or the like). AA, or any airline, will be happy to sell you a B-A-B ticket if your A-B-A was a walk-up full-fare (Y) purchase.

So, just to answer your question, both of your last two scenarios are legal tickets.

For all practical purposes, most airlines don't have any way to track this. Even with back-to-backs, you'll usually be fine as long as you buy the tickets from two different sources (e.g. one from your travel agent and one from AA.com). Again, this explanation is not to be interpreted as my condoning such behavior.

If you have any more questions, head over to the Mileage Run or UA fora and search on "back-to-back" or on posts by the aforementioned guru keithguy. Hope this helps!

Mook

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The future of US domestic air travel:
"Please watch your head as you exit the aircraft."


[This message has been edited by Mook (edited 01-30-2003).]